Zinc Oxide Composites with

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One-pot synthesis of noble metal/zinc oxide composites with controllable morphology and high catalytic performance Zhihong Bao, Yue Yuan, Chunbo Leng, Li Li, Kun Zhao, and Zhenhua Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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ACS Applied Materials & Interfaces

One-pot Synthesis of Noble Metal/Zinc Oxide Composites with Controllable Morphology and High Catalytic Performance Zhihong Bao,‡*a Yue Yuan,‡a Chunbo Leng,c Li Li,c Kun Zhao,a and Zhenhua Sun*b

a b

School of Pharmacy, Shenyang Pharmaceutical University, 110016 Shenyang, P. R. China. Shenyang National Laboratory for Material Science, Institute of Metal Research, Chinese

Academy of Sciences, 110016 Shenyang, P. R. China. c

School of Chemistry and Molecular Engineering, East China Normal University, 500

Dongchuan Road, 200241 Shanghai, P. R. China.

KEYWORDS: one-pot synthesis, Pd/ZnO composite, Pt/ZnO composite, controllable morphology, catalysis.

ABSTRACT The combination of noble metal and oxide-support is a good approach to reduce the cost of noble metal catalyst and improve the stability of nanocatalyst in chemical reactions. Here, noble metal/zinc oxide composites, including Pd/ZnO and Pt/ZnO, have been facilely prepared through the general one-pot hydrothermal method. Importantly, the morphology of composites can be tuned from tube, flower, and star, to skin needling-like shapes by easily adjusting the alkalinity of the reaction solution. By investigating the growth mechanism and influencing factors of the

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morphology of noble metal/zinc oxide composites, the difference of the morphology of composites can be ascribed to various growing units and directions of ZnO crystal under different alkalinities. Among them, Pd/ZnO composites exhibited both the higher catalytic activity and the excellent stability in the Suzuki coupling reaction between iodobenzene and phenylboronic acid and the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride. Such supported composites will potentially be used as green and sustainable catalysts for many chemical reactions.

1. INTRODUCTION The nanoscale noble metals exhibit fascinating optical, electronic, magnetic, and catalytic properties that are often radically different from their bulk counterparts.1–4 These novel properties stem from the quantum size effect and surface effect.5–7 However, the high surface energy of noble metal nanocatalysts leads to easily agglomerate or undergoes morphological change during catalytic reactions, resulting in a dramatic decrease of their activity and selectivity.8 For the development of green, sustainable, and economical chemical processes, the rational design and synthesis of high efficient, stable, reusable, and economical catalysts become the major goal in current catalysis research.9,10 Therefore, there emerge many novel strategies, including control of the structure and composition of the active nanoparticles (NPs) and the manipulation of the interaction between the catalytically active NPs species and their support to prevent agglomeration and growth of active NPs. Among the above strategies, supported nanocatalysts have been demonstrated to reduce the surface energies of noble metal NPs, as well as maintain or improve their stability and reusability in catalytic reactions. Metal-support interactions can also influence certain properties of NPs, one of which is the alteration of metal


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electronic properties through bringing contact of NPs with support. This process could alter the adsorption affinity of the metal towards the reactants and/or products as a result could affect its catalytic activity. Typically, the supports for noble metal NPs are often based on activated carbon or metal oxides. Liu et al. reported a template-based procedure for the fabrication of carbon nanotubes with Pd NPs being uniformly embedded in the inner carbon surfaces. The as-prepared Pd/C nanocomposite possessed a high recyclability in a liquid-phase Suzuki coupling reaction.11 Chen et al. developed a new non-noble metal sacrificial approach for the immobilization of highly dispersed AgPd NPs on reduced graphene oxide. The resulting ultrafine AgPd NPs exhibited the high activity for the dehydrogenation of formic acid for generation of hydrogen without CO impurity.12 Recently, a novel ZnO support-induced encapsulation strategy was utilized to fabricate a Pd/ZnO@ZIF-8 core-shell catalyst, with Pd/ZnO as the core and ZIF-8 as the shell. The as-prepared Pd/ZnO@ZIF-8 core-shell microsphere can be employed as an efficient catalyst that displayed excellent performance in terms of size-selectivity, stability and anti-poisoning in the liquid hydrogenations of alkenes.13 Several works revealed that the morphologies and structures of support strongly influenced the size and dispersion of NPs, as well as the interaction between NPs and supports which was closely related to the catalytic activity.14–18 Cui et al. achieved the depositing of Cu on CeO2-support with different shape facets in a two-step process. It has been revealed that the effect of the shape/crystal planes is a key factor that can influence the interactions between Cu species and CeO2 supports.19 Luo et al. for the first time reported CeOx-TiO2 catalysts with three morphologies, e.g., ceria from clusters to nanochains and nanoparticles. Furthermore, photocatalytic tests revealed that the catalyst with small low-dimensional ceria clusters and high dispersity resulted the best activity for photocatalytic water splitting.20 At present, most supported metallic composites were fabricated


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through physical mixing,21,22 electrochemical deposition,23 noncovalent interactions,24,25 and coprecipitation. However these methods lacked the fine control of the support morphology, and most of them required complex preparation processes. In addition, due to the relatively weak interaction between the metal and the support surfaces, these composites were stable only under certain chemical conditions. Recent work revealed that co-growth method could enhance the adhesion between loading NPs and support, thus improve the stability of such composites. The co-growth strategy is usually conducted under hydrothermal process. There were several advantages for using hydrothermal method, such as relatively simple, high-yield, and highcrystallinity etc. More importantly, the shape, size, and crystalline properties could be controlled through the simple alteration of reaction temperature, pressure, ration of reactants, and reaction pH, all of which were also crucial for tailoring specific properties of these materials. Compared to other catalyst support, ZnO with its nontoxicity, lower cost, and abundant easy-to-prepare morphologies as support is well-known to induce the strong metal-support interaction with noble metal, which offers the opportunities for tuning material properties to create specific sites that influence the catalytic behavior.26 In the present work, we reported the synthesis of supported noble metal-ZnO composites with superior catalytic efficiencies through a simple hydrothermal route with metal acetylacetonate as the precursors, polyvinyl pyrrolidone (PVP) as the stabilizing agent, and N, Ndimethylformamide (DMF) as reduction agent. Importantly, the morphology of ZnO support in the composites could be tailored from tube, flower, star, to skin needling by simply adjusting the reaction pH. Since several reactants were reacted synchronously, all reactants were able to well mix and disperse in the reaction system. During the hydrothermal treatment, strong interaction of noble metal NPs with support material occurred in the prepared composites. Compared with


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previous reported multi-step reaction methods, the growth of composites and loading process of noble metal NPs took place simultaneously in one pot in our approach, which could avoid the antagonism between noble metal NPs and support and could form the strong interaction of noble metal NPs with support in composites. We also demonstrated that capability of our approach to prepare other noble metal/zinc oxide (Pt/ZnO). The detailed mechanism for morphological evolution of the noble metal-zinc oxide composites was discussed in detail. The influence of synthetic parameters on the supported morphology and loading amount of noble metal have been investigated. Specifically, the Pd/ZnO composites showed favorable catalytic activity and good recyclability in both the Suzuki coupling reaction and the reduction of 4-nitrophenol (4-NP). The universality of our synthesis method and investigation of catalytic properties will therefore provide a convenient paradigm for the design and synthesis of versatile catalysts for applications in the green and sustainable chemical processes. 2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials Zinc acetylacetonate (Zn(acac)2), platinum acetylacetonate (Pt(acac)2), phenylboronic acid, iodobenzene and biphenyl were purchased from Alfa Aesar. Palladium acetylacetonate (Pd(acac)2) was purchased from Sigma-Aldrich. Sodium hydroxide (NaOH), polyvinyl pyrrolidone, potassium phosphate, N, N-dimethylformamide, 4-nitrophenol (4-NP) and acetone were purchased from Sinopharm Chem. Reagent Co., Ltd. All chemicals were directly used without further purification. Deionized water (18.2 MΩ) was used throughout experiment. 2.2 Synthesis of Pd/ZnO composites Typically, Pd(acac)2 (1.5 mg, 0.005 mmol), Zn(acac)2 (60 mg, 0.228 mmol), PVP (0.1 g) were added into solvent (DMF:H2O 10 mL:2 mL), followed by the addition of NaOH solution (0.25


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M) to adjust the pH of the solution. The volume of NaOH solution was 0, 200, 1700, and 2500 µL, respectively. The mixed solution was stirred until the reactants were well dispersed. The resulting solution was transferred into a Teflon-lined, 25 mL autoclave, and heated at 140 °C for 2 h, and then cooled to room temperature. Finally, after adding a few drops of acetone into the above reaction product, the solution was centrifuged at 5000 rpm for 10 min and then redispersed in solvent (ethanol/acetone v:v 1:1). The solid product was washed with ethanol/acetone two times by ultrasonication for 10 min and centrifuged for 5000 rpm (10 min), and oven dried at 50‒60 °C for further using. 2.3 Synthesis of Pt/ZnO composites The synthesis method for Pt/ZnO composites was similar to that of Pd/ZnO composites except that Pd(acac)2 was replaced by Pt(acac)2 in synthesis process. Pt(acac)2 (1 mg, 0.003 mmol), Zn(acac)2 (60 mg, 0.228 mmol), PVP (0.1 g) were added into solvent (DMF:H2O 10 mL:2 mL), followed by the addition of NaOH solution (0.25 M). The volume of NaOH in the solution was also 0, 200, 1700, and 2500 µL, respectively. The solution was stirred until the reactants were well dispersed. The resultant dispersive mixture was thereafter transferred into a Teflon-lined, 25 mL autoclave and heated at 140 °C for 2 h and then taken out for natural cooling in air. The treatment method of product was the same to that was used for preparing Pd/ZnO composites. 2.4 Characterization Scanning electron microscopy (SEM) images were acquired on an FEI Quanta 400 FEG microscope. Transmission electron microscopy (TEM) images were performed on an FEI CM120 microscope. High-resolution transmission electron microscopy (HRTEM) and highangle annular dark field-scanning transmission electron microscopy (HAADF-STEM) characterizations were carried out on an FEI Tecnai F20 microscope equipped with an Oxford


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energy-dispersive X-ray analysis system. X-Ray diffraction (XRD) patterns were collected on an X-ray diffractometer (D/max 2500/PC, JEOL Ltd.) with Cu Kα radiation (λ = 1.5418 Å). Nitrogen adsorption-desorption isotherms were recorded by using a Micromeritics ASAP 2020 surface area and porosity analyzer. The mass percent of Pd and ZnO was measured by inductively coupled plasma optical emission spectra (ICP-OES) analysis (PerkinElmer Optima 8300, PerkinElmer Technologies, USA). The gas chromatography (GC) analysis was performed on an Agilent 7890A GC system, which equipped with an HP-5 MS capillary column. The ultraviolet-visible (UV-Vis) spectra were measured on a 4802H UV/Vis/NIR double beam spectrophotometer (Unico Instrument Co. Ltd., Shanghai, China). 2.5 Catalytic performance test The Suzuki coupling reaction between phenylboronic acid and iodobenzene was performed using as-prepared Pd/ZnO composites as catalyst. In a typical reaction, iodobenzene (0.034 mL, 0.3 mmol) was added to mixing solution (EtOH: H2O = 4:1 v:v, 12 mL) in the presence of phenylboronic acid (0.486 g, 3.98 mmol), K2PO4•3H2O (2.13 g, 8 mmol), and powder Pd/ZnO catalyst (30 mg). The mixture was heated in an oil bath at 85 °C under gentle stirring for 30 min. The product was added to CHCl3 solution, followed by the centrifugation at 8000 rpm for 10 min. The supernatant was analyzed by GC analysis to obtain the yield of biphenyl. The precipitation containing Pd/ZnO catalyst was washed by ethanol and water for two times for recycling the catalyst. A series of experiments were carried out under the same procedure, except that the Pd/ZnO composites with different morphologies were utilized. According to the results of ICP-OES analysis (Table 1), the Pd contents in 30 mg of tube-like, flower-like, star-like and skin needling-like Pd/ZnO composites were 1.02 mg, 1.00 mg, 0.92 mg and 0.8 mg, respectively.


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The reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was also performed using as-prepared Pd/ZnO composites as catalyst. In a typical reaction, the powder Pd/ZnO composites (5 mg) was dispersed in water (1 mL) in advance. First, the aqueous solutions of 4NP (0.01 M, 0.03 mL) and NaBH4 (0.5 M, 0.2 mL) were added into deionized water (2.5 mL) in a quartz cuvette with stirring. Then 0.02 mL of the well-dispersed Pd/ZnO catalyst was injected into the reaction solution. The net weight of the catalyst sample in the reaction solution was 0.1 mg. After the reaction solution was stirred for 10 s, the quartz cuvette was transferred into a UVVis spectrophotometer for monitoring the change in absorbance at 400 nm as a function of time. According to the results of ICP-OES analysis, 0.1 mg of tube-like, flower-like, star-like and skin needling-like Pd/ZnO catalysts contained 3.39 µg, 3.35 µg, 3.08 µg and 2.67 µg Pd, respectively. For testing the recyclability, the reaction solution was composed of 4-NP (0.01 M, 0.3 mL), NaBH4 (0.5 M, 0.3 mL), and tube-like Pd/ZnO composites (5 mg/mL, 0.2 mL). The reaction progress was monitored by taking out 0.02 mL of the solution at 2 min intervals, diluting it with water (1.8 mL), and measuring the absorption spectrum. At the end of each cycle, the catalyst were precipitated by centrifugation and re-dispersed in water (0.2 mL), and were transferred to a fresh mixture of same reactant. 3. RESULTS AND DISCUSSION 3.1 Synthesis and structural characterization of Pd/ZnO composites In this work, the noble metal/zinc oxide composites were prepared by a one-step hydrothermal method in solution, which employed Zn(acac)2 and Pd(acac)2 as precursors, DMF and NaOH solution (0.25 M) as alkali source and reductant, and PVP as the capping agent. Firstly, Zn(acac)2, Pd(acac)2, and PVP were added into a beaker containing a mixture of DMF and deionized water. A certain amount of NaOH aqueous solution (0.25 M) was subsequently added.


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The final solution was magnetically stirred until complete dispersion at room temperature, followed by transferring into a Teflon-lined autoclave and heat treatment at 140 °C for 2 h. As the reaction proceeded, the color of the mixture changed from ivory to grey black. After washing and drying, the black solid product was obtained. The preparation process for the Pd/ZnO composites was illustrated in Scheme 1. The mass ratio of Pd to ZnO was about 2.9 % according to the calculation result of weight ratio of Pd(acac)2 to Zn(acac)2 in hydrothermal reaction.

Scheme 1. Illustration of the synthesis of noble metal/zinc oxide composites. Figure 1a and 1b show the low-magnification SEM and HAADF-STEM images of the prepared tube-like Pd/ZnO composites, which was treated without NaOH and the pH of reaction solution was ~ 6. The length of these nanotubes typically varied from 200 nm to 350 nm with diameters from 50 nm to 150 nm. The shape of nanotubes was uniform with one end of nanotubes closed. The HRTEM analysis was then conducted for verifying the crystalline structure of the as-prepared tube-like Pd/ZnO composites. Figure 1c clearly shows that the Pd NPs were well dispersed on the ZnO supports and the HRTEM images were acquired from


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marked square regions on tube-like composites. Lattice fringes were clearly observed and were ascribed to well-defined (111) plane of Pd and (002) plane of wurtzite-type ZnO, respectively (Figure 1d). Figure 1e displays a HAADF-STEM image of tube-like Pd/ZnO composites, on which elemental mapping analysis was conducted. Figure 1f–1h show STEM-energy dispersive X-ray (STEM-EDX) elemental maps of Zn, O and Pd, respectively. The different colors correlates the presence of different elements, where pink refers to Zn, blue refers to O, and orange refers to Pd. Taken together, these results revealed that the support was mainly composed of Zn and O, and Pd NPs with diameter of 4~8 nm only anchored on the surface of ZnO support.

Figure 1. The tube-like Pd/ZnO composites obtained by hydrothermal treatment at 140 °C for 2 h (pH ~ 6). (a) SEM image, (b) STEM image, (c) TEM and (d) HRTEM images of one tube-like


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composites. (e) HAADF-STEM image and (f–h) energy dispersive X-ray (EDX) elemental maps of Zn, O and Pd, respectively.

When the pH of the reaction solution was increased to 7 (200 µL of 0.25 M NaOH solution was injected into the reaction solution), the tubes of the tube-like ZnO-supports tended to be slim rods and the branch number was also increased. Many slim and long branches were overlapped and interweaved together. Finally, the morphology of the products was tuned into flower-like and the size of the particles was increased to micrometer level (Figure 2a and 2b). The range of the flower-like particle size is around 1.5~3 µm, as measured from their SEM and TEM images. The crystalline structure of the flower-like Pd/ZnO composites was also characterized with HRTEM (Figure 2c). The HRTEM image in Figure 2d acquired from marked region on one branch of the flower-like composites in Figure 2c. The clearly-resolved lattice fringes were also ascribed to (002) plane of crystalline ZnO and (111) plane of Pd. In addition, HRTEM image reveals that diameter of Pd NPs was 14~20 nm. The obtained HAADF-STEM image and EDX elemental maps shows that Pd NPs were well-dispersed on ZnO support (Figure 2f–h).


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Figure 2. (a) SEM image and (b) TEM image of the flower-like Pd/ZnO composites obtained by hydrothermal treatment at 140 °C for 2 h (pH ~ 7). (c) TEM and (d) HRTEM images of one branch of flower-like composites. (e) HAADF-STEM image. (f–h) EDX elemental maps of Zn, O and Pd, respectively.


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Figure 3 shows star-like Pd/ZnO composites, which were obtained under the experiment condition of the 1700 µL, 0.25 M NaOH and pH ~ 9. The size of Pd/ZnO composites was ~500 nm. As shown in Figure 3c, the Pd NPs on the surface were clearly observed. These composites were similar to the reported star-like ZnO/Au and ZnO/Ag nanohybrids using surfactant (cetyltrimethylammonium bromide, CTAB) as a growth controlling agent, and strong base (hydrazine hydrate) as a reducing agent.27 Analogous composites were obtained in this work under similar experiment conditions. As the reaction pH value exceeding 10 (the 2500 µL, 0.25 M NaOH was injected in to the reaction solution), the branches of star-like Pd/ZnO nanostructure became more slender and branchy, which was termed skin needling-like Pd/ZnO (Figure S1a). Figure S2 displays the large range SEM images of four Pd/ZnO composites, suggesting the high yield and well dispersion of composites. The crystal structure of the Pd/ZnO composites was further examined by the XRD patterns (Figure 4). All XRD diffraction peaks of the four Pd/ZnO composites could be indexed as a combination of the typical wurtzite structure of ZnO (JCPDS 36-1451) and the face-centered-cubic structure of Pd (JCPDS 46-1043), and no other crystalline impurities were observed, which were consistent with above HRTEM results. In contrast, it was difficult to observe any signal from Pd NPs in all of the four composites. This is likely due to the small size of Pd NPs in four composites compared to support ZnO particles, as verified by the broad and weak diffraction peaks in the XRD patterns. The specific surface area and porosities of the tube-like, flower-like and star-like Pd/ZnO composites have been further characterized by nitrogen adsorption analyses. As can be seen from the nitrogen adsorptiondesorption isotherm plots and pore size distributions (Figure S3), these Pd/ZnO composites exhibit high specific surface (25‒33 m2 g-1) and hierarchical pore structures (mesopore and macropore), which are beneficial to increase the contact of the catalyst with the reactants and


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thus to enhance the catalytic activity. The amount of Pd and ZnO and the actual mass percent of Pd and ZnO in four Pd/ZnO composites were determined by ICP-OES analysis. Table 1 shows that the measured mass percent of Pd in each composite (2.67%–3.5%) were generally larger or closed to theoretical mass percent of Pd (~ 2.9%) based on the experimental dosages of Pd(acac)2 and Zn(acac)2, indicating that the addition of Pd(acac)2 to the precursor solution was almost completely transferred into Pd NPs and incorporated into the metal oxide support by our present method. Moreover, the results of ICP-OES also showed that the decrease of the relative Pd loading amount in Pd/ZnO composites was caused by the increase of the alkaline of reaction solution, which is likely because that the fast decomposition of Pd precursor was inhibited and the nuclei numbers were reduced under high alkali condition during the nucleation stage. As a result, relative few and larger Pd NPs tended to be formed when the concentration of NaOH was increased in the reaction solution.

Figure 3. The star-like Pd/ZnO composites obtained by hydrothermal treatment at 140 °C for 2 h (pH ~ 9). (a) SEM image. (b,c) TEM images.


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Figure 4. The XRD patterns of the tube-like (a), flower-like (b), star-like (c), and skin needlinglike (d) Pd/ZnO composites. The red color belongs to the wurtzite structure of ZnO and blue color belongs to the face-centered-cubic structure of Pd, respectively. Table 1. ICP-OES measured the actual mass of Pd and ZnO, and the mass percent of Pd in each Pd/ZnO composites. Sample

Pd (mg/L)

Tube-like Pd/ZnO Flower-like Pd/ZnO Star-like Pd/ZnO Skin needling-like Pd/ZnO

0.426 0.524 0.806 0.504

Zn (mg/L) 9.733 12.129 20.325 14.748

The mass percent of Pd in each composites (%) 3.39 % 3.35 % 3.08 % 2.67 %

3.2 The applicability of the synthesis method We further produced other noble metal/zinc oxide composites (Pt/ZnO) using similar hydrothermal method. The synthetic route was similar to that of Pd/ZnO composites except that Pd source was replaced by Pt source. Figure 5 and Figure S1b show representative SEM and TEM images of Pt/ZnO composites in reaction conditions with different alkalinities. When reaction solution was treated without any NaOH, Pt/ZnO composites were in tube shape with


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closed one-end (Figure 5a, b) that was more curve compared to the tube-like Pd/ZnO structures. HRTEM image (Figure S4) taken from an individual Pt/ZnO nanotube exhibited lattice fringes corresponding to the (002) plane of ZnO. Due to the stronger electron scattering ability of Pt than ZnO, the Pt NPs appeared darker in the HRTEM image. However, Pt NPs were very small (~ 1 nm) in this composites, therefore lattice fringes of Pt NPs cannot be observed in HRTEM image. The EDX elemental maps of a single tube-like Pt/ZnO composite (Figure 5c–f) revealed that the small Pt NPs were uniformly distributed in the ZnO particles. When the pH value of the reaction solution was increased to about 7 (the 200 µL, 0.25 M NaOH was injected into the reaction solution), the flower-like Pt/ZnO composites were also obtained (Figure 5g, h). The image obtained by HAADF-STEM (Figure 5h) clearly revealed the homogeneous distribution of small Pt NPs in the ZnO particles. When the alkalinity was continuously increased, the star-like Pt/ZnO composites were obtained (Figure 5i, j). When the alkalinity was increased to pH~10, the skin needling-like Pt/ZnO composites were generated (Figure S1b). Based on the above results, this one-step hydrothermal method could be generally applied to the preparation of many noble metal/zinc oxide composites with controlled morphology.


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Figure 5. (a) The SEM image. (b) TEM image and (c) HAADF-STEM image of tube-like Pt/ZnO composites obtained by hydrothermal treatment at 140 °C for 2 h (pH ~ 6). (d–f) EDX elemental maps of Zn, O and Pt, respectively. (g) The SEM image and (h) STEM image of flower-like Pt/ZnO composites obtained by hydrothermal treatment at 140 °C for 2 h (pH ~ 7). (i) The SEM image and (j) TEM image of star-like Pt/ZnO composites obtained by hydrothermal treatment at 140 °C for 2 h (pH ~ 9). 3.3 Formation mechanism of the noble metal/zinc oxide composites


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Based on the above results, the effect of reaction pH on the morphology of composites was clear. As shown in Figure 1 and Figure 5a‒f, without the aid of NaOH, a tube shape was formed in the composites. At the presence of NaOH solution with low concentration, the flower shape was formed (Figure 2 and Figure 5g, h). Gradually increasing the concentration of the NaOH in the reaction solution, different shapes could then be formed: from the star-like to the skin needling-like Pd/ZnO composites (Figure 3, Figure 5i, j and Figure S1, respectively). Thus, the control of the shape of the composites could be achieved by easily varying the amount of the NaOH in the reaction solution. Furthermore, to help understand the effect of alkalinity on the morphology of composites we reason the synthetic mechanism as detailed below. Firstly, without using NaOH, the alkalinity of the reaction solution mainly stemmed from solvent DMF, which was relatively low (pH ~ 6). During hydrothermal processing, the precursor of the zinc source was first decomposed to Zn2+, and the generation of a large number of Zn2+ turned the reaction solution into an over saturated state, followed by the nucleation and growth of ZnO according to the equation (1). Therefore, Zn2+ ion was the “growing unit” for ZnO nanocrystal under low alkalinity condition (pH ~ 6). According to the reported literature28, the growth of ZnO took place in the (002) direction, finally leaded to tube-like shape. Zn2+ + 2OH– → ZnO + H2O (1) When NaOH solution (200 µL, 0.25 M) was added to reaction solution (pH ~ 7), OH– initially reacted with Zn2+ ion and formed Zn(OH)2 according to the equation (2). During hydrolysis at 140 °C, Zn(OH)2 colloid partially dissolved into Zn2+ and OH– cresting the ZnO nuclei. On the other hand, Zn(OH)2 further reacted with OH– and as a result [Zn(OH)4]2– was formed according to the equation (3). Thus, [Zn(OH)4]2– as the new “growing unit”, grew along (002) direction of


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ZnO crystal nuclei and formed slim and long ZnO nanorods. Finally, many slim and long ZnO nanorods were interweaved together and the flower-like composites were eventually generated. Zn2+ + 2OH– → Zn(OH)2 → ZnO + H2O (2) Zn(OH)2 + 2OH– → [Zn(OH)4]2– (3) [Zn(OH)4]2– → ZnO + 2H2O +2OH– (4) At pH ~ 9, the formation of the composites also followed the above equations (2), (3), (4). [Zn(OH)4]2– acted as the “growing unit” for the formation of ZnO composites. Owing to the high concentration of OH– under experiment condition, more [Zn(OH)4]2– were formed and concentrated along the (002) direction of ZnO crystal nuclei according to the equation (3). Similarly, due to sufficient [Zn(OH)4]2– and excessive negatively charged [Zn(OH)4]2– the growth in other directions became relatively faster than the (002) direction, thus the forming ZnO nanorods became thick and short, finally the star-like structures were obtained. In addition, according to the literature29 free hydroxyl groups (provided by alkaline solution) were protonated and conjugated with Pd2+ or Pt2+ via electrostatic attraction in reaction process. Then Pd2+ or Pt2+ were reduced by reducing agent (e.g. DMF and NaOH solution) and were nucleated in the ZnO support. Although the reaction alkalinity was a necessity for the formation of ZnO support and loading of noble metal, but high alkalinity could also decrease the loading amount of Pd and ZnO because of the increased amount of produced ZnO (Table 1). Thus, the pH of reaction system was adjusted to meet the needs for the morphology control and application of the composites. In case of surfactant assisted synthesis of ZnO, surfactant plays a crucial role and acts as a driving force for the formation of ZnO nanoflower.30 Therefore, to explore the role of PVP in the formation of composites with different morphologies, we carried out control experiments without


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PVP. Figure S5 shows the SEM image of Pd/ZnO using 0 µL (Figure S5a, b) and 200 µL (Figure S5c, d) NaOH (0.25 M) without PVP, where all composites were rambling. On the contrary, with sufficient PVP (see Figure 1 and 2), crystal growth was restricted in certain directions on the ZnO nuclei, we therefore concluded that PVP was the directing agent and growth inhibitor. In addition, the PVP was also found to play an important role in controlling the size and dispersity of loading metal NPs. As illustrated in Figure 1 and 2, a certain amount of PVP could sustain a suitable growth rate of Pd for uniform size and high dispersion, while without PVP the loading Pd NPs grew much larger (~100 nm) and agglomerated on ZnO support (Figure S5). The above discussion well demonstrated that PVP was essential for the formation of ZnO support with different morphologies and loading of noble metal NPs. On the other hand, the morphology of the nanostructures prepared under other fixed conditions without the addition of the metal precursor was very similar to those of the noble metal/zinc oxide composites (Figure S6). Taken together, the reaction pH and surfactant were major factors affecting the morphology of composites and loading of noble metal NPs. 3.4 The catalytic performances of Pd/ZnO composites The Suzuki coupling reaction, in which Pd is utilized as the catalyst, plays a very crucial role in the chemical industry.31 Over the past decade, Pd nanocrystals had been prepared in a variety of different shapes and sizes for maximizing the catalytic activity of Pd nanocrystals. Among them, Jin et al. reported the synthesis of palladium concave nanocubes with high-index (730) facets through preferential overgrowth on Pd cubic seeds.32 Compared with the conventional Pd nanocubes enclosed by low-index (100) facets, the Pd concave nanocubes exhibited substantially enhanced catalytic activities in the Suzuki coupling reaction. However, when pure Pd NPs were used as the catalytic, high catalytic efficiency was also accompanied by an increase in the cost of


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catalysis in chemical reactions. Recently, composites-catalysts with low cost, easy separation, recyclability, and reusability have attracted great attention. To investigate the catalytic performance of our Pd/ZnO composites, the Suzuki coupling between iodobenzene and phenylboronic acid was employed as model reaction. For each catalyst, the coupling reaction was carried out three times under the same conditions. Figure 6a‒b display the reaction equation of the Suzuki coupling reaction between iodobenzene and phenylboronic acid, and the yields of biphenyl product and the cycle performance of Pd/ZnO catalysts, respectively. For four Pd/ZnO catalysts with an equal amount, a conversion yield reached 99% after reaction for 30 min. In comparison, a conversion yield of 38% was obtained for conventional Pd nanocubes.32,33 In addition, the yield of biphenyl was still above 95% after three cycles (Figure 6b). The excellent catalytic stability was attributed to the existence of strong adhesion strength between ZnOsupport and Pd NPs through co-growth hydrothermal treatment, which prevented the desorbing of the Pd NPs from the as-prepared Pd/ZnO catalysts during the catalytic process. We also compared the catalytic efficiency of the catalyst with those reported elsewhere of Pd-based heterogeneous catalysts. It can be seen that our Pd/ZnO catalyst exhibited a better catalytic efficiency in comparison to reported Pd/C, Pd/Fe3O4 catalysts (Table S1, entry 4, 5, 6). Although some of them also obtained high yields (Table S1, entry 2, 3), but some toxic solvent (such as 1methyl-2-pyrrolidinone, anisole, 1,2-dimethoxyethane) were used, while the used solvents (EtOH and H2O) are more favourable in this work. Thus, the Pd/ZnO composites as catalyst seemed to be the best candidate in this work, showing an excellent efficiency for the Suzuki cross-coupling reactions in terms of yield, cost, reusability and facile preparation.


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Figure 6. (a) The equation of the Suzuki coupling reaction between phenylboronic acid and indobenzene. (b) Yields of biphenyl and cycle performance using different Pd/ZnO composites as the catalyst in the Suzuki coupling reaction. The composites catalytic properties of the present Pd/ZnO composites were also evaluated by a typical model reaction: the reduction of 4-nitrophenol to 4-aminophenol with NaBH4 aqueous solution at room temperature. It was known that this reduction reaction did not occur in the absence of catalysts due to the kinetic barrier caused by the large potential difference between the donor (BH4–) and the acceptor molecules (4-nitrophenol).34 Metal NPs (Au, Pd) could catalyze the reaction by acting as an electronic relay to overcome the kinetic barrier, allowing electron transfer from BH4– to 4-nitrophenol.35 4-nitrophenol initially exhibited an absorption peak at 317 nm in neutral or acidic solution. The addition of NaBH4 could deprotonate the OH group of 4-NP. Due to the formation of the 4-nitrophenolate ion,36,37 the solution colour changed from colourless to yellow and the absorption peak was shifted to 400 nm. The addition of Pd/ZnO catalyst caused a decolorization of the 4-nitrophenolate solution, meanwhile the


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intensity of the characteristic absorption peak of 4-nitrophenol at 400 nm quickly dropped until disappeared within 12 min along with the appearance of two new adsorption peaks at 233 and 300 nm (Figure 7b), suggesting the formation of 4-aminophenol (4-AP),38,39 and the complete reduction of 4-nitrophenol to 4-aminophenol without the formation of byproducts. Based on the Beer-Lambert Law, the reaction kinetics could be investigated by monitoring the absorbance at 400 nm as a function of reaction time. It should be noted that all experiments were carried out with fixed concentrations of 4-nitrophenol (1.2 × 10–4 M) and NaBH4 (4 × 10–2 M). Because the concentration of NaBH4 was comparatively enormous compared with 4-nitrophenol and always excess during the whole reaction, it was considered as a constant. Thus, the reduction reaction of 4-nitrophenol to 4-aminophenol could be assumed to be the pseudo-first-order reaction. Figure 7c shows the linear plot of ln[C(t)/C(0)] against reaction time, where C(t) and C(0) were converted from the peak absorbance at 400 nm at time t and 0, respectively. The pseudo-first-order rate constant k, obtained from the rate equation ln[C(t)/C(0)] = kt and the slope of above linear fit result, was 0.348 min–1 (tube-like Pd/ZnO composites). The time-dependent absorption spectra and reaction kinetics of the other 3 catalyst samples were shown in Figure S7. The pseudo-firstorder rate constants k were achieved to be 0.47, 0.324, and 0.26 min–1 for reactions catalyzed by the flower-like, the star-like, and the skin needling-like Pd/ZnO composites, respectively. Clearly, the catalytic performance of flower-like Pd/ZnO catalyst was superior than other Pd/ZnO catalysts. These results indicated that slim and long branches in flower-like composites were beneficial due to the formation of highly dispersed Pd with small particle size (in good agreement with the result in the TEM and HRTEM images of Figure 2) and the synergistic effect of NPs with support was substantially enhanced. The catalytic activities of tube-like and star-like Pd/ZnO catalysts were relatively similar, which agrees well with previous discussion on


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morphology (Figure 1 and 3). To compare our result with literature values, we calculated the catalytic activity k’ (k’ = k/mPd), where mpd is the total mass of the palladium added as catalyst. The catalytic activity for four Pd/ZnO composites were ~ 1700 s−1g−1, higher than those reported Pd-based catalysts (Table S2). Compared with Pd NPs, the Pd/ZnO composite have the merits of less aggregation during the catalytic reaction due to the interaction between NPs and support, and the catalysts could be easily separated from the reaction system. In order to investigate the recyclability and stability of the Pd/ZnO composites, the cycle performance test was carried out. As shown in Figure 7d and S8, the catalysts were still highly active after four cycles. With increasing cycles the complete conversion of 4-NP could be achieved by increasing reaction time from 4 min to 6 min for tube-like Pd/ZnO composites. The extension of reaction time may be due to the loss of small portions of the catalyst during centrifugation and re-dispersion in the recycling process. These results indicated that the Pd/ZnO composites exhibited high catalytic activity and good recyclability, which promote widespread applications of the catalyst.

Figure 7. (a) The equation of reduction reaction of 4-nitrophenol to 4-aminophenol. (b) Timedependent absorption spectra of the reaction solution in the presence of the tube-like Pd/ZnO


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composites. (c) Plot of ln[C(t)/C(0)] against the reaction time. (d) Plot of ln[C(t)/C(0)] against time in four cycles of reduction with catalyst.

4. CONCLUSIONS In summary, hydrothermal method has been employed for the synthesis of supported noble metal/zinc oxide composites with four interesting morphologies, including tube-like, flower-like, star-like, and skin needling-like. Our studies on the formation mechanism revealed that both alkalinity and surfactant were crucial on controlling the formation of well-defined noble metalzinc oxide composites. The as-prepared composites displayed superior catalytical capability and recyclability in the Suzuki coupling reaction and the reduction of 4-nitrophenol. Among these catalysts, the flower-like Pd/ZnO composites showed optimal catalytic performance in the catalytic reduction of 4-nitrophenol. The different structures of ZnO led to the different particle sizes and dispersities of supported Pd. We believed that this facile and general method could be further applied to the fabrication of various recyclable, low-cost, and highly efficient supported catalysts. ASSOCIATED CONTENT Supporting Information. SEM images, TEM images, HRTEM images and the experimental results for catalytic reduction of 4-nitrophenol. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author Zhihong Bao: [email protected]. Zhenhua Sun: [email protected].


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNPWLEDGEMENTS We thank J. F. Wang’s group for high-resolution and high-angle annular dark field-scanning TEM. This study was financially supported by the National Natural Science Foundation of China (Grant No. 21401132 and 21505045), Shanghai Pujiang Program (15PJ1401800), Natural Science Foundation of Liaoning Province of China (Grant No. 201602688 and 201602702) and Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2015150).

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Table of Contents Graphic


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Zinc Oxide Composites with

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