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ZnO nanorods/ZnS$(1,6-hexanediamine)0.5 hybrid nanoplates hierarchical heteroarchitecture with improved electrochemical catalytic properties for hydrazine† Zhengcui Wu,* Yaqin Wu, Tonghui Pei, Huan Wang and Baoyou Geng* Novel hierarchical heteronanostructures of ZnO nanorods/ZnS$(HDA)0.5 (HDA ¼ 1,6-hexanediamine) hybrid nanoplates on a zinc substrate are successfully synthesized on a large scale by combining hydrothermal growth (for ZnO nanorods) and liquid chemical conversion (for ZnS$(HDA)0.5 nanoplates) techniques. The formation of ZnS$(HDA)0.5 hybrid nanoplates branches takes advantage of the preferential binding of 1,6-hexanediamine on specific facets of ZnS, which makes the thickening rate much lower than the lateral growth rate. The ZnS$(HDA)0.5 hybrid nanoplates have a layered structure with 1,6-hexanediamine inserted into interlayers of wurtzite ZnS through the bonding of nitrogen. The number density and thickness of the secondary ZnS$(HDA)0.5 nanoplates can be conveniently engineered by variation of the sulfur source and straightforward adjustment of reactant concentrations
Received 1st October 2013 Accepted 27th November 2013
such
as
1,6-hexanediamine
and
the
sulfur
source.
The
fabricated
ZnO/ZnS$(HDA)0.5
heteronanostructures show improved electrochemical catalytic properties for hydrazine compared with DOI: 10.1039/c3nr05231a
the primary ZnO nanorods. Due to its simplicity and efficiency, this approach could be similarly used to
www.rsc.org/nanoscale
fabricate varieties of hybrid heterostructures made of materials with an intrinsic large lattice mismatch.
Introduction Heterostructured nanomaterials with desirable spatial organization of diverse materials are being designed and constructed to meet ever-increasing technological requirements.1 Extensive investigations have been directed toward the controlled synthesis of such smart heteronanostructures for making devices with integrated multi-functionality of disparate components or enhanced and even novel properties, not otherwise achievable with any of the individual components alone or with their physical mixture counterparts. ZnO and ZnS, as important group II–VI semiconductors, are promising materials for wide applications in sensors, lasers, eld emitters, nanogenerators, solar cells, photocatalysis, and so on.2–4 It has been demonstrated by both theoretical calculations and experimental results that ZnO/ZnS heterostructures can exhibit superior properties such as luminescence, photocatalysis, photovoltaic, and electrochemistry compared to their individual components.5–18 During the past decade, there have been
Anhui Key Laboratory of Molecule-Based Materials, The Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People's Republic of China. E-mail: [email protected]; [email protected]; Fax: +86-553-3869302; Tel: +86-553-3869302 † Electronic supplementary 10.1039/c3nr05231a
information
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(ESI)
available.
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extensive efforts to optimize synthesis strategies for component- and structural-modulated synthesis of ZnO/ZnS heterostructures for a variety of applications. However, most conventional syntheses are mainly related to polycrystalline ZnS nanoparticles covered on ZnO without a crystallographic epitaxial relationship,18–23 restricting their potential highperformance applications. The limited ZnO/ZnS heterostructures with single crystalline ZnS shells are mostly based on high temperature vapor deposition growth techniques,5,7–10,16,24–29 which extremely restrict their large-scale production and application due to the critical conditions such as high reaction temperature, tedious processes, and complex devices required. Therefore, it is of great interest to explore facile and rational solution-phase synthetic strategies for obtaining structure- and orientation-regulated ZnO/ZnS heterostructures with high yields as well as for the resulting property improvements. However, the related report was so rarely,12 due to their intrinsic large lattice mismatch along the interfaces and high sensitivity to synthetic conditions. Recently, inorganic–organic hybrid materials aroused a lot of interest for new properties and technological applications.30 Incorporation of two counterparts into a single structure may combine or improve the superior properties of inorganic framework, such as electronic, magnetic, and optical properties, rigidity, and thermal stability with that of organic molecules, e.g., the structural diversity, exibility, processability, and
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geometrical controllability.31 II–VI hybrid semiconductors containing II–VI slabs, chains or 1D nanocrystals incorporating various amines in one structure via coordinate or covalent bonds have been developed,31b,32,33 which has a composition of [MQ(L)x] (M ¼ Mn, Zn, Cd; Q ¼ S, Se, Te; L ¼ mono- or diamine or hydrazine; x ¼ 0.5 or 1.0), where nanosized components of identical size are arranged in a perfectly periodic crystal structure.31b,32,34 Further developments in rational synthetic strategies for II–VI based hybrid semiconductors with other amines especially solid water-soluble alkylamine will be particularly attractive not only for their structural uniformity and periodicity but also for new functional devices. Metal chemical corrosion and oxidation approach has been designed for the synthesis of ZnO branched nanostructures on zinc substrates.35 Herein, we further developed this strategy, demonstrating a rst example of ZnS$(HDA)0.5 hybrid nanoplates (ZnS$(HDA)0.5-NPs) epitaxially grown on ZnO nanorods (ZnO-NRs) to form a hierarchical heterostructure on zinc substrate with ZnO-NRs as backbones and epitaxial growth of ZnS$(HDA)0.5-NPs as branches via a facile two-step liquid-phase approach, combining hydrothermal deposition and chemical conversion processes. The ZnS$(HDA)0.5 hybrid nanoplates have a layered structure with 1,6-hexanediamine inserted into interlayers of wurtzite ZnS through the bonding of nitrogen. Excitingly, the density and thickness of the branches can be easily tuned through the variation of the sulfur source as well as adjustment of reactant concentrations such as 1,6-hexanediamine and the sulfur source, which is very attractive for practical applications. Moreover, the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures exhibited superior electrochemical sensing properties for hydrazine in comparison with ZnO-NRs, which can be attributed to increased current conduction and surface area of the heterostructure. This work is important for advancing fundamental understanding of ZnO-NRs/ ZnS$(HDA)0.5-NPs heteronanostructures and engineering novel functional devices, as well as providing the possibility for designing varieties of orientation-regulated hybrid heteronanostructures made of materials with an intrinsic large lattice mismatch.
Experimental section Synthesis of ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructures on a zinc substrate Firstly, ZnO-NRs on a zinc substrate were synthesized according to our recent report.36 Briey, 4 mL of 1,6-hexanediamine was dissolved in 40 mL of distilled water in a Teon-lined stainless steel autoclave. Then a cleaned zinc foil (99.9% 1 3 cm) was inserted at an angle into the solution. The autoclave was sealed and maintained at 180 C for 5 h, then allowed to cool to room temperature naturally. The white precipitate covering both sides of the zinc foil was washed with distilled water and ethanol for several times, and dried in a vacuum at 50 C for 6 h. Secondly, sodium sulde nonahydrate (Na2S$10H2O, 1.1 mmol) was added into a mixed solvent of 1,6-hexanediamine (2 mL) and distilled water (58 mL) to form a homogenous solution under constant stirring in a 100 mL glass bottle, followed by
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insertion of the zinc foil covered by ZnO-NRs. The bottle was loaded into an oven and maintained at 60 C for 8 h. The zinc foil covered with white precipitate was washed three times with distilled water and ethanol, and dried in a vacuum at 50 C for 6 h (named as Sample 1). The other two kinds of ZnS$(HDA)0.5 nanoplate-like branches on ZnO-NRs backbones were synthesized with thioacetamide and thiosemicarbazide (1.1 mmol each) as the sulfur sources while keeping other parameters constant except for the addition of 4 mL of 1,6-hexanediamin and loaded into a Teon-lined stainless steel autoclave and maintained at 120 C when using thiosemicarbazide as the sulfur source (named as Sample 2 and Sample 3, respectively). Sample characterization X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku TTRIII diffraction system with highintensity Cu Ka radiation. Field emission scanning electron microscope (FESEM) images were obtained on a Hitachi S-4800 eld-emission scanning electron microscope operated at an accelerating voltage of 5.0 kV. Transmission electron microscopy (TEM) analysis was obtained using a JEOL 2010 with an accelerating voltage of 200 kV. The Fourier transform infrared (FTIR) spectroscopic study was carried out with a MAGNA-IR 750 (Nicolet Instrument Co.) FTIR spectrometer. Elemental analyses data were obtained on a Vario EL III elemental analyzer. Electrochemical experiments were performed using a CHI660B electrochemical analyzer (ChenHua Instruments Co. Ltd., Shanghai, China) with a conventional three-electrode system. The working electrode was the as-prepared ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure with a naonmodied glassy carbon electrode (Sample/Nf/GCE). A saturated calomel electrode and a platinum wire electrode were used as the reference and the auxiliary electrode, respectively. Electrode preparation The fabrication of the electrode of the sample was followed as according to the literaure.37 Briey, ten milligrams of the ZnO/ ZnS$(HDA)0.5 heteronanostructure detached from the surface of the zinc foil by ultrasonication in ethanol and dried in a vacuum was dispersed in 10 mL of 0.5% naon solution in N,N-dimethylformamide by ultrasonic treatment. Next, 5 mL of the suspension was dropped onto the surface of cleaned GCE and dried in air. Prior to each electrochemical experiment, solutions were purged with puried nitrogen for 15 min to remove oxygen, and maintained under a nitrogen atmosphere during the course of the experiments.
Results and discussion The crystal structure and phase composition of ZnO-NRs/ ZnS$(HDA)0.5-NPs heteronanostructure as well as ZnO-NRs were rst revealed by XRD analysis. The result demonstrates that the wurtzite-type ZnO-NRs can be directly developed on the zinc substrate (Fig. S1†). Aer the second liquid-phase treatment, the diffraction peaks with corresponding lattice spacings of 1.338, 0.670 and 0.438 nm at low angles can be indexed as (002),
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(004) and (006), respectively (Fig. 1). Two additional diffraction peaks appeared with d values of 0.320 nm and 0.307 nm and these can be indexed to the (102) and (103) crystal planes of hexagonally-structured wurtzite ZnS with 8H symmetry (JCPDS no. 72-0163). The diffraction peaks appearing at low angles is one of the characteristics of the inorganic–organic hybrid composites such as II–VI slabs incorporated various amines, e.g., ZnS(en)0.5 (en ¼ ethylenediamine).38 Here, the diffraction peaks presenting with increased d values in low angles demonstrated that 1,6-hexanediamine molecules have inserted into the interlayer of ZnS. The XRD pattern indicated that the as-synthesized sample is composed of ZnO and a ZnS–1,6-hexanediamine hybrid. The morphology and composition of the nal product were characterized by the FESEM with energy dispersive X-ray spectroscopy (EDX). The FESEM images and EDX show the product aer the rst hydrothermal treatment was needle-like nanorods with an atomic ratio of Zn and O of 1 : 1 (Fig. S2†). Fig. 2a and b show low and high magnication FESEM images of the heterostructure aer the second liquid-phase treatment. It can be clearly seen that the secondary nanoplates are uniformly assembled on the whole side surface of the primary ZnO-NRs and the nanoplates are exible with a lateral size of ca. 280 nm. From the compositional information of the point-scan EDX in Fig. 2c, an S element is found to be present along with Zn and O, providing powerful evidence that the S element is successfully incorporated into the ZnO-NRs surface. Further elemental analysis by combustion for the product indicates that the material includes C 6.09%, H 1.62%, N 2.42%, and S 6.04%. Accordingly, the atomic ratio of C, H, N and S was 6.0 : 19.2 : 2.0 : 2.2, demonstrating 1,6-hexanediamine was inserted into the product in the molecule form (the slightly higher content of hydrogen was due to the adsorption of water molecules in surroundings), and the possible formula of ZnS hybrid material can be determined as ZnS$(HDA)0.5. Moreover, the product was composed of about 70.7% ZnO and 29.3% ZnS
Fig. 1 XRD pattern of the as-prepared ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructure, which is obtained by detaching the sample from the zinc foil by ultrasonication in ethanol and drying it in a vacuum.
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Morphology and structural characterization of the ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure. ((a) and (b)) Low and high magnification FESEM images, (c) EDX spectrum, (d) TEM image of an individual ZnO-NRs/ZnS$(HDA)0.5-NPs, (e) magnified TEM image of a part of individual ZnO-NRs/ZnS$(HDA)0.5-NPs, showing the flexible and nearly transparent nanoplate structure, and (f) HRTEM image of the junction, which shows the epitaxial growth relationship of the ZnS nanoplate on the ZnO nanorod. Fig. 2
Fig. 3 FESEM images of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures synthesized with other sulfur sources. ((a) and (b)) The product with thioacetamide as the sulfur source, and ((c) and (d)) the product with thiosemicarbazide as the sulfur source.
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Fig. 4 XRD patterns of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures synthesized with other sulfur sources. (a) The product with thioacetamide as the sulfur source, and (b) the product with thiosemicarbazide as the sulfur source.
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Scheme 1 Schematic image of the fabrication of ZnO-NRs/ ZnS$(HDA)0.5-NPs heterostructure on a zinc substrate. (a) Small ZnS hybrid nanoplates covered on the side surfaces of needle-like ZnONRs (step 1), and more intensive and larger lateral size nanoplates generated on thinner ZnO-NRs, forming the final ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure (step 2), and (b) the proposed formation mechanism of the ZnS$(HDA)0.5 nanoplates. For clarity, hydrogen atoms are omitted in the structure of 1,6hexanediamine.
Fig. 5 FTIR spectra of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures. ((a)–(c)) Samples 1, 2, 3, respectively.
hybrid (according to the S element analysis). The detailed crystallinity and composition of as-synthesized hierarchical heteronanostructure were further examined by TEM measurements. A typical TEM image of an individual ZnO-NRs/ ZnS$(HDA)0.5-NPs heteroarchitecture in Fig. 2d further clearly reveals that exible and nearly transparent ZnS$(HDA)0.5-NPs are attached on ZnO-NR, and the backbone of ZnO-NR was about 2.8 mm long and 330 nm wide near the half height of the nanorod, which is ner than that of pure ZnO-NRs (about 380 nm wide). This indicated the formation of ZnS$(HDA)0.5NPs is from the consumption of ZnO-NRs. The magnied TEM image of a part of individual ZnO-NRs/ZnS$(HDA)0.5-NPs in Fig. 2e further presented the exible and thin nanoplate structure of ZnS$(HDA)0.5 on ZnO-NR. The HRTEM image at the edge of the nanoplate shows the continuous lattice fringes (Fig. S3†), demonstrating its single-crystal nature. The corresponding fast Fourier transform (FFT) spot diagram (Fig. S3† inset) further clearly veries the single-crystal nature of the nanoplate. The
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6 CV performances of bare GCE and the ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure of a Sample 3 modified electrode in the absence and presence of N2H4 in 0.1 M PBS of pH 7.0 at a scan rate of 50 mV s1. Fig.
HRTEM image in Fig. 2f clearly identied the great dark/bright contrast along both sides of the interface, where different crystal parts are displayed. Although some stacking faults and other crystal defects are present in the interfacial region, both the backbone and branch exhibit clear lattice fringes. One set of the planar spacing was about 0.26 nm, which corresponded to the (002) plane of hexagonal wurtzite-type ZnO. Another set of the fringes' spacing measured ca. 0.31 nm, corresponding to the (008) lattice spacing of hexagonal ZnS, indicating that the ZnS
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Fig. 7 Electrochemical impedance spectroscopy of different electrodes, which were obtained in 0.5 M KCl solution containing 5.0 mM Fe(CN)63/Fe(CN)64.
Fig. 8 CV performances of different ZnO-NRs/ZnS$(HDA)0.5-NPs and ZnO-NRs modified electrodes in the presence of 200 mM N2H4 in 0.1 M PBS of pH 7.0 at a scan rate of 50 mV s1.
Fig. 9 Typical amperometric responses of the ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure of Sample 3-modified electrode with successive addition of 50 mM N2H4 in 0.1 M PBS of pH 7.0 at 0.5 V.
nanoplate branch was indeed grown over the ZnO nanorod backbone. Upon further examination of the interface, an
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epitaxial growth relationship between the ZnO-NRs and ZnSNPs component is also suggested, that is [002]ZnOk[008]ZnS. The HRTEM image reveals the good single-crystalline nature of the ZnS nanoplate, which preserves the crystal structure characteristics and follows the crystallographic orientation of the initial ZnO nanorod. Therefore, the secondary ZnS hybrid nanoplates have a layered structure in which 1,6-hexanediamine acts as the interlayer molecules connecting ZnS layers. Investigation of the reaction conditions, such as the reaction time, the concentrations of 1,6-hexanediamine and Na2S, and other sulfur resources on the microstructures of secondary ZnS hybrid nanoplates, have been carefully investigated. The morphology evolution of the intermediates involved in the formation caught at 10 min, 30 min and 60 min showed some tiny nanoplates covered on the nanorods aer 10 min (Fig. S4a†), and with time prolonged to 30 min, the product was larger lateral size nanoplates covered on the nanorods (Fig. S4b†). At 60 min, the nanoplates with much larger lateral size and more intensive structure on ZnO-NRs are observed (Fig. S4c†). And at 8 h, large exible nanoplate-like structure uniformly covered on ZnO-NRs is obtained (Fig. 2a and b). As the reaction time was further increased to 24 h, the morphology of the heterostructure is barely unchanged (Fig. S4d†). Timeresolved experiments revealed that the suldation process began at the surface of the ZnO-NRs with some small nanoplates formed rst, which gradually expanded, and generated more intensive and larger lateral size nanoplates on ner ZnONRs. When the growth achieved dynamic equilibrium at a certain reaction stage, the microstructure of the heterostructure remained basically unchanged with further aging. The further investigations on the concentration effects of 1,6-hexanediamine and Na2S showed that as the amount of 1,6hexanediamine increased to 4 mL while keeping other parameters constant, hierarchical heterostructures with more ZnS hybrid nanoplates on ZnO-NRs were obtained (Fig. S5a†), demonstrating that a moderate increase of 1,6-hexanediamine will favor the conversion of ZnO-NRs into ZnS hybrid nanoplates. But, when further increased to 6 mL, the product was obviously less regular (Fig. S5b†), which may be due to the overhigh concentration of 1,6-hexanediamine promoting the rapid formation of ZnS hybrid nanoplates branches at the rapid dissolution of ZnO-NRs backbones. In addition, when the Na2S concentration was doubled, more intensive ZnS hybrid nanoplates were covered on the ZnO-NRs (Fig. S5c†), indicating that the more sulfurous the source is, the more transformations the ZnS hybrid nanoplates on ZnO-NRs will have. These results suggest that the elegant ZnO-NRs/ZnS-HDA-NPs hierarchical hybrid heterostructures can be obtained at suitable concentrations of 1,6-hexanediamin and Na2S. It is worth noting that the strategy for obtaining hierarchical ZnS-HDA-NPs on ZnO-NRs by 1,6-hexanediamin-assisted synthesis is not limited to Na2S as the sulfur source but also can be extended to other sulfur sources, such as thioacetamide and thiosemicarbazide. When using thioacetamide as the sulfur source, the ZnS-HDA-NPs covering the ZnO-NRs surface were thicker (Fig. 3a and b). The corresponding TEM image (Fig. S6a†) further revealed the hierarchical nanoplates covering
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the nanorod, and the HRTEM image from the junction (Fig. S6b†) also displayed two types of clear lattice fringes, which can be indexed to the (002) and (008) lattice spacing of single crystalline hexagonal ZnO and ZnS, respectively. While using thiosemicarbazide as the sulfur source, the nanoplates grown on the side surfaces of ZnO-NRs become longer and ner and form an elegant ower at the top of the ZnO-NRs (Fig. 3c and d). The TEM images (Fig. S6c and d†) and HRTEM images (Fig. S6e and f†) further showed the hierarchical nanostructure and single-crystalline nature of the ZnS-NPs. The corresponding XRD patterns shown in Fig. 4 were similar to Sample 1. In addition, the small-angle diffraction peaks shi slightly towards lower angles for Sample 3, demonstrating that the d-spacing of the planes increased. The elemental analysis showed the products include 9.77% C, 2.41% H, 4.04% N and 9.46% S for Sample 2, and 10.91% C, 2.78% H, 4.22% N and 9.80% S for Sample 3. Further analysis reveals the atomic ratio of C, H, N and S was 6.0 : 17.8 : 2.1 : 2.2 for Sample 2, while it was 6.0 : 18.4 : 2.0 : 2.0 for Sample 3. The possible formula of the ZnS hybrid materials can still be determined as ZnS$(HDA)0.5. Meanwhile, the products were composed of about 54.1% ZnO and 45.9% ZnS hybrid for Sample 2 and 52.4% ZnO and 47.6% ZnS hybrid for Sample 3 (according to the S element analysis). The higher elemental contents demonstrated more ZnS hybrid nanoplates were formed from the consumption of ZnO-NRs, which is consistent with the results of FESEM and TEM. The further investigations on the concentrations of 1,6-hexanediamine and corresponding sulfur sources give us more opportunities to tune the microstructure of ZnS-HDA-NPs covered on ZnO-NRs (Fig. S7†). The experimental results also suggest that the temperature is another important parameter affecting the microstructures of secondary ZnS-HDA-NPs (Fig. S8†). To elucidate the effect of the zinc substrate on the formation of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructure, an experiment using ZnO-NRs detached by ultrasonication from the zinc substrate was performed in the second liquid-phase conversion process with other parameters kept constant. Aer the reaction, ZnO-NRs/ZnS$(HDA)0.5-NPs were still generated, but the product was poorly dened (Fig. S9†). The result implied that the zinc substrate plays an important role for obtaining welldened ZnO-NRs/ZnS$(HDA)0.5-NPs. In our case, the aligned needle-like ZnO-NRs stood on a zinc substrate possess a high distribution density and a wide separation between the neighboring nanorods, supplying a good opportunity for the uniform conversion of ZnS$(HDA)0.5-NPs on the large exposed surfaces of ZnO-NRs via the second liquid-phase chemical conversion process, whose similar role has been demonstrated on ZnONRs/Ag-NPs heterostructures.36 In order to clarify the effect of 1,6-hexanediamine on realizing the formation of hierarchical ZnS$(HDA)0.5-NPs branches, an experiment with NH3$H2O instead was performed in the second conversion step. Aer reaction, only tiny ZnS nanoparticles covered on the ZnO-NRs were obtained (Fig. S10†). The results demonstrated that the 1,6-hexanediamine plays a particular role on the formation of the ZnS$(HDA)0.5-NPs branches.
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In this approach, the formation of ZnS$(HDA)0.5-NPs on ZnO-NRs is due to the suldation conversion in the second liquid-phase growth process through the reaction of ZnO-NRs with the sulfur source in the presence of 1,6-hexanediamine. However, what role does 1,6-hexanediamine play in terms of controlling crystal growth? Being a kind of water-soluble alkylamine with a bidentate ligand, two lone pairs of electrons associated with the two nitrogen atoms interact with cationic zinc ions. At the beginning of the suldation reaction, S2 anions released from the decomposition of the sulfur source at elevated temperature react with the Zn2+ slowly dissolved from the surface of ZnO-NRs, producing ZnS nanoparticles around the ZnO-NRs. Further, 1,6-hexanediamine preferentially bonded on the ZnS surface perpendicular to the (001) facet through surface ions, and thus passivated the surface atoms, resulting in the ZnS crystal preferring to grow along [001] and [010] directions into 2D nanostructure. At the same time, 1,6hexanediamine molecules are incorporated into neighboring ZnS layers by coordination. As the suldation time increased, the nanoplates further grow and generate uniform and dense ZnS$(HDA)0.5 nanoplates’ branches. Thus, ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure gradually form. Therefore, it is the preferential binding of 1,6-hexanediamine on specic facets of ZnS that makes the thickening rate much lower than the lateral growth rate; meanwhile, 1,6-hexanediamine molecules were inserted into the interlayers of ZnS by coordination of N atoms with neighboring layers in a vertically aligned mode, leading to the formation of 2D ZnS$(HDA)0.5-NPs covered on ZnO-NRs. The Fourier transform infrared (FTIR) spectra of the heterostructure samples conrmed the coordination of 1,6-hexanediamine in the products (Fig. 5). The sharp absorption peaks observed at 3242 cm1 and 3125 cm1 were the stretching vibration of the amino group, which are shied to the lower frequencies (usually at 3500–3300 cm1) due to the binding of the amino group in the heterostructure. The absorption peaks observed at 2930 cm1 and 2844 cm1 were associated with the stretching vibration of a C–H bond in methylene. Meanwhile, the band at 1470 cm1 and 1357 cm1 can be indexed to the bending vibrational mode of C–H bond in methylene. The absorption peaks observed at 1615 cm1 and 1515 cm1 were the bending vibration of N–H bond in amino group. The bands at 1072 cm1 and 996 cm1 are attributed to the stretching vibration of C–N bond. Therefore, the FTIR spectra conrmed the existence of 1,6hexanediamine in the heterostructures, which supported our mechanism from another point of view. The microstructure variation of ZnS$(HDA)0.5-NPs in the heterostructure with changes of the sulfur source may be associated with the difference of the concentration of S2 decomposed from the sulfur source during the reaction process at the designed temperature and the assisted coordinated role of the sulfur source. Of course, it still needs more detailed and systematic work to provide evidence to make clear the precise functions of 1,6-hexanediamine and the synergistic effect of the sulfur source on the shape control of ZnS$(HDA)0.5-NPs branches. Based on the experimental results, the formation mechanism of ZnO-NRs/ ZnS$(HDA)0.5-NPs is schematically shown in Scheme 1.
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The synthesized heterostructure products have been investigated for electrochemical catalysis of hydrazine to evaluate their chemical sensing properties. Fig. 6 shows the cyclic voltammograms of the bare GCE and the ZnO-NRs/ZnS$(HDA)0.5NPs hierarchical heterostructure of a Sample 3 modied electrode in the absence or presence of hydrazine. Comparing with the bare GCE, a weak redox peak appeared on Sample 3/Nf/GCE, which proved that the prepared ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heteroarchitecture was successfully modied onto the GCE. When the hydrazine was added, the Sample 3/Nf/GCE electrode exhibited an intense electrocatalytic activity for hydrazine oxidation. The catalytic oxidation is sharp and occurs at about 0.2 V, reecting a faster electron-transfer reaction owing to the high catalytic effect of the special heteronanostructure. However, the oxidation of hydrazine requires very high positive overpotentials at bare GCE, leading to a poorly dened anodic wave characteristic of very slow electrode kinetics. In order to study the electrocatalytic activity of different ZnONRs/ZnS$(HDA)0.5-NPs hierarchical heterostructures, the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) spectra were obtained for the samples. Electrochemical impedance spectroscopy (EIS) can provide useful information on the impedance changes of the electrode surface during the fabrication process. The Nyquist plot of the EIS includes a semicircular portion and a linear portion. The semicircular portion at higher frequencies corresponds to the electron-transfer limited process and its diameter is equal to the electron transfer resistance (Ret), which controls the electron transfer kinetics of the redox probe at the electrode interface. Meanwhile, the linear part at lower frequencies corresponds to the diffusion process.39 Fig. 7 displays the Nyquist plots of the EIS of different electrodes. Aer three different ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures (Samples 1 to 3) and ZnO-NRs were attached onto the GCE, the semicircle diameter of EIS, Ret, is different for the different electrodes. The EIS was in a sequence with Ret ZnO > Ret Sample 2 > Ret Sample 1 > Ret Sample 3, which meant that the electron transfer ability is Sample 3 > Sample 1 > Sample 2 > ZnO. Obviously, the Ret ZnO is much larger than that of three kinds of ZnO-NRs/ ZnS$(HDA)0.5-NPs heterostructures, indicating the conductivity of the heterostructure is much better than ZnO-NRs. Herein, the increased conductivity of ZnO-NRs/ZnS$(HDA)0.5-NPs may be attributed to the intimate mixing at the interface of the ZnO-NRs and ZnS$(HDA)0.5-NPs, which forms excess Zni donors due to bond rearrangement by the electronegativity difference between S and O.18 Because Zni is known to act as shallow donors in ZnO,40 the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructure becomes more conducting. The ZnO-NRs/ZnS nanoparticles core–shell nanostructure synthesized in the presence of NH3$H2O appeared as a partially-dissolved ZnO core/ZnS shell nanostructure (Fig. S10c†), the Ret of the ZnO-NRs/ZnS nanoparticles is even larger than that of ZnO-NRs (Fig. S11†). Meanwhile, the formation of the ZnO-NRs/ ZnS$(HDA)0.5-NPs heterostructure obviously increases the surface area of the product, which also makes the electrochemical probe arrive at the electrode's surface easily. The electrochemical responses of hydrazine on three different ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructures
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and ZnO-NRs were further investigated (Fig. 8). All the three hierarchical heterostructures have an improving effect on the response of hydrazine compared with ZnO-NRs, and the largest increment of the peak current and most well-dened peak with the same concentration of hydrazine appeared on Sample 3. The superior electrochemical sensing property of the heterostructure can be explained by the improved conductance and increased surface area. The results also showed the microstructure of ZnONRs/ZnS$(HDA)0.5-NPs had a noticeable inuence on their electrochemical catalytic properties, which can be mainly attributed to the variations in surface area. The sample with a larger surface area enables more reactive-sites for electrocatalysis, improving the corresponding sensing performance. The differences in the CV results were consistent with the variations of EIS. Fig. 9 shows a typical amperometric response of the hierarchical heterostructure of Sample 3 to the successive addition of hydrazine (50–1000 mM) into continuously stirred 0.1 M PBS (pH ¼ 7.0) at an applied potential of 0.5 V. The amperometric sensor exhibited a rapid and sensitive response to the change of hydrazine concentration with the oxidation current obviously increased upon successive addition of hydrazine. An inset in Fig. 9 shows the corresponding calibration curve of the fabricated amperometric hydrazine sensor. The results indicate that the ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructuresmodied electrodes have a potential application for N2H4 chemosensor.
Conclusion In summary, we describe a simple liquid-phase chemical conversion reaction with ZnO nanorods on a zinc substrate to produce ZnO nanorods/ZnS$(HDA)0.5 hierarchical heterostructures with a tunable number density and thickness of the secondary ZnS$(HDA)0.5 nanoplate-like structure. In this selfassembly process, the preferential binding of 1,6-hexanediamine on specic facets of ZnS plays a particular role for the formation of the hybrid ZnS$(HDA)0.5 nanoplates’ branches and accounting for the insertion of 1,6-hexanediamine molecules into interlayers of ZnS. The as-synthesized hierarchical heteroarchitectures exhibited superior electrochemical catalytic properties for N2H4, which can be attributed to the better conductivity and larger surface area of the heterostructures. It is undoubtedly only one of their properties, leaving the expanded chemical functionality and physical properties to be explored. Meanwhile, this simple and efficient two-step liquid-phase strategy with preferential binding of water-soluble alkylamine for the orientation-regulated hierarchical hybrid nanoheterostructures is expected to allow the fabrication of other complex semiconductor hybrid heterostructures with an intrinsic large lattice mismatch, thus opening up new avenues for bringing on series of unprecedented excellent properties.
Acknowledgements This work was nancially supported by the Natural Science Foundation of China (21201007 and 21271009), the Program for New Century Excellent Talents in University (NCET 11-0888),
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ZnO nanorods/ZnS$(1,6-hexanediamine)0.5 hybrid nanoplates hierarchical heteroarchitecture with improved electrochemical catalytic properties for hydrazine† Zhengcui Wu,* Yaqin Wu, Tonghui Pei, Huan Wang and Baoyou Geng* Novel hierarchical heteronanostructures of ZnO nanorods/ZnS$(HDA)0.5 (HDA ¼ 1,6-hexanediamine) hybrid nanoplates on a zinc substrate are successfully synthesized on a large scale by combining hydrothermal growth (for ZnO nanorods) and liquid chemical conversion (for ZnS$(HDA)0.5 nanoplates) techniques. The formation of ZnS$(HDA)0.5 hybrid nanoplates branches takes advantage of the preferential binding of 1,6-hexanediamine on specific facets of ZnS, which makes the thickening rate much lower than the lateral growth rate. The ZnS$(HDA)0.5 hybrid nanoplates have a layered structure with 1,6-hexanediamine inserted into interlayers of wurtzite ZnS through the bonding of nitrogen. The number density and thickness of the secondary ZnS$(HDA)0.5 nanoplates can be conveniently engineered by variation of the sulfur source and straightforward adjustment of reactant concentrations
Received 1st October 2013 Accepted 27th November 2013
such
as
1,6-hexanediamine
and
the
sulfur
source.
The
fabricated
ZnO/ZnS$(HDA)0.5
heteronanostructures show improved electrochemical catalytic properties for hydrazine compared with DOI: 10.1039/c3nr05231a
the primary ZnO nanorods. Due to its simplicity and efficiency, this approach could be similarly used to
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fabricate varieties of hybrid heterostructures made of materials with an intrinsic large lattice mismatch.
Introduction Heterostructured nanomaterials with desirable spatial organization of diverse materials are being designed and constructed to meet ever-increasing technological requirements.1 Extensive investigations have been directed toward the controlled synthesis of such smart heteronanostructures for making devices with integrated multi-functionality of disparate components or enhanced and even novel properties, not otherwise achievable with any of the individual components alone or with their physical mixture counterparts. ZnO and ZnS, as important group II–VI semiconductors, are promising materials for wide applications in sensors, lasers, eld emitters, nanogenerators, solar cells, photocatalysis, and so on.2–4 It has been demonstrated by both theoretical calculations and experimental results that ZnO/ZnS heterostructures can exhibit superior properties such as luminescence, photocatalysis, photovoltaic, and electrochemistry compared to their individual components.5–18 During the past decade, there have been
Anhui Key Laboratory of Molecule-Based Materials, The Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, People's Republic of China. E-mail: [email protected]; [email protected]; Fax: +86-553-3869302; Tel: +86-553-3869302 † Electronic supplementary 10.1039/c3nr05231a
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extensive efforts to optimize synthesis strategies for component- and structural-modulated synthesis of ZnO/ZnS heterostructures for a variety of applications. However, most conventional syntheses are mainly related to polycrystalline ZnS nanoparticles covered on ZnO without a crystallographic epitaxial relationship,18–23 restricting their potential highperformance applications. The limited ZnO/ZnS heterostructures with single crystalline ZnS shells are mostly based on high temperature vapor deposition growth techniques,5,7–10,16,24–29 which extremely restrict their large-scale production and application due to the critical conditions such as high reaction temperature, tedious processes, and complex devices required. Therefore, it is of great interest to explore facile and rational solution-phase synthetic strategies for obtaining structure- and orientation-regulated ZnO/ZnS heterostructures with high yields as well as for the resulting property improvements. However, the related report was so rarely,12 due to their intrinsic large lattice mismatch along the interfaces and high sensitivity to synthetic conditions. Recently, inorganic–organic hybrid materials aroused a lot of interest for new properties and technological applications.30 Incorporation of two counterparts into a single structure may combine or improve the superior properties of inorganic framework, such as electronic, magnetic, and optical properties, rigidity, and thermal stability with that of organic molecules, e.g., the structural diversity, exibility, processability, and
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geometrical controllability.31 II–VI hybrid semiconductors containing II–VI slabs, chains or 1D nanocrystals incorporating various amines in one structure via coordinate or covalent bonds have been developed,31b,32,33 which has a composition of [MQ(L)x] (M ¼ Mn, Zn, Cd; Q ¼ S, Se, Te; L ¼ mono- or diamine or hydrazine; x ¼ 0.5 or 1.0), where nanosized components of identical size are arranged in a perfectly periodic crystal structure.31b,32,34 Further developments in rational synthetic strategies for II–VI based hybrid semiconductors with other amines especially solid water-soluble alkylamine will be particularly attractive not only for their structural uniformity and periodicity but also for new functional devices. Metal chemical corrosion and oxidation approach has been designed for the synthesis of ZnO branched nanostructures on zinc substrates.35 Herein, we further developed this strategy, demonstrating a rst example of ZnS$(HDA)0.5 hybrid nanoplates (ZnS$(HDA)0.5-NPs) epitaxially grown on ZnO nanorods (ZnO-NRs) to form a hierarchical heterostructure on zinc substrate with ZnO-NRs as backbones and epitaxial growth of ZnS$(HDA)0.5-NPs as branches via a facile two-step liquid-phase approach, combining hydrothermal deposition and chemical conversion processes. The ZnS$(HDA)0.5 hybrid nanoplates have a layered structure with 1,6-hexanediamine inserted into interlayers of wurtzite ZnS through the bonding of nitrogen. Excitingly, the density and thickness of the branches can be easily tuned through the variation of the sulfur source as well as adjustment of reactant concentrations such as 1,6-hexanediamine and the sulfur source, which is very attractive for practical applications. Moreover, the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures exhibited superior electrochemical sensing properties for hydrazine in comparison with ZnO-NRs, which can be attributed to increased current conduction and surface area of the heterostructure. This work is important for advancing fundamental understanding of ZnO-NRs/ ZnS$(HDA)0.5-NPs heteronanostructures and engineering novel functional devices, as well as providing the possibility for designing varieties of orientation-regulated hybrid heteronanostructures made of materials with an intrinsic large lattice mismatch.
Experimental section Synthesis of ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructures on a zinc substrate Firstly, ZnO-NRs on a zinc substrate were synthesized according to our recent report.36 Briey, 4 mL of 1,6-hexanediamine was dissolved in 40 mL of distilled water in a Teon-lined stainless steel autoclave. Then a cleaned zinc foil (99.9% 1 3 cm) was inserted at an angle into the solution. The autoclave was sealed and maintained at 180 C for 5 h, then allowed to cool to room temperature naturally. The white precipitate covering both sides of the zinc foil was washed with distilled water and ethanol for several times, and dried in a vacuum at 50 C for 6 h. Secondly, sodium sulde nonahydrate (Na2S$10H2O, 1.1 mmol) was added into a mixed solvent of 1,6-hexanediamine (2 mL) and distilled water (58 mL) to form a homogenous solution under constant stirring in a 100 mL glass bottle, followed by
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insertion of the zinc foil covered by ZnO-NRs. The bottle was loaded into an oven and maintained at 60 C for 8 h. The zinc foil covered with white precipitate was washed three times with distilled water and ethanol, and dried in a vacuum at 50 C for 6 h (named as Sample 1). The other two kinds of ZnS$(HDA)0.5 nanoplate-like branches on ZnO-NRs backbones were synthesized with thioacetamide and thiosemicarbazide (1.1 mmol each) as the sulfur sources while keeping other parameters constant except for the addition of 4 mL of 1,6-hexanediamin and loaded into a Teon-lined stainless steel autoclave and maintained at 120 C when using thiosemicarbazide as the sulfur source (named as Sample 2 and Sample 3, respectively). Sample characterization X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku TTRIII diffraction system with highintensity Cu Ka radiation. Field emission scanning electron microscope (FESEM) images were obtained on a Hitachi S-4800 eld-emission scanning electron microscope operated at an accelerating voltage of 5.0 kV. Transmission electron microscopy (TEM) analysis was obtained using a JEOL 2010 with an accelerating voltage of 200 kV. The Fourier transform infrared (FTIR) spectroscopic study was carried out with a MAGNA-IR 750 (Nicolet Instrument Co.) FTIR spectrometer. Elemental analyses data were obtained on a Vario EL III elemental analyzer. Electrochemical experiments were performed using a CHI660B electrochemical analyzer (ChenHua Instruments Co. Ltd., Shanghai, China) with a conventional three-electrode system. The working electrode was the as-prepared ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure with a naonmodied glassy carbon electrode (Sample/Nf/GCE). A saturated calomel electrode and a platinum wire electrode were used as the reference and the auxiliary electrode, respectively. Electrode preparation The fabrication of the electrode of the sample was followed as according to the literaure.37 Briey, ten milligrams of the ZnO/ ZnS$(HDA)0.5 heteronanostructure detached from the surface of the zinc foil by ultrasonication in ethanol and dried in a vacuum was dispersed in 10 mL of 0.5% naon solution in N,N-dimethylformamide by ultrasonic treatment. Next, 5 mL of the suspension was dropped onto the surface of cleaned GCE and dried in air. Prior to each electrochemical experiment, solutions were purged with puried nitrogen for 15 min to remove oxygen, and maintained under a nitrogen atmosphere during the course of the experiments.
Results and discussion The crystal structure and phase composition of ZnO-NRs/ ZnS$(HDA)0.5-NPs heteronanostructure as well as ZnO-NRs were rst revealed by XRD analysis. The result demonstrates that the wurtzite-type ZnO-NRs can be directly developed on the zinc substrate (Fig. S1†). Aer the second liquid-phase treatment, the diffraction peaks with corresponding lattice spacings of 1.338, 0.670 and 0.438 nm at low angles can be indexed as (002),
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(004) and (006), respectively (Fig. 1). Two additional diffraction peaks appeared with d values of 0.320 nm and 0.307 nm and these can be indexed to the (102) and (103) crystal planes of hexagonally-structured wurtzite ZnS with 8H symmetry (JCPDS no. 72-0163). The diffraction peaks appearing at low angles is one of the characteristics of the inorganic–organic hybrid composites such as II–VI slabs incorporated various amines, e.g., ZnS(en)0.5 (en ¼ ethylenediamine).38 Here, the diffraction peaks presenting with increased d values in low angles demonstrated that 1,6-hexanediamine molecules have inserted into the interlayer of ZnS. The XRD pattern indicated that the as-synthesized sample is composed of ZnO and a ZnS–1,6-hexanediamine hybrid. The morphology and composition of the nal product were characterized by the FESEM with energy dispersive X-ray spectroscopy (EDX). The FESEM images and EDX show the product aer the rst hydrothermal treatment was needle-like nanorods with an atomic ratio of Zn and O of 1 : 1 (Fig. S2†). Fig. 2a and b show low and high magnication FESEM images of the heterostructure aer the second liquid-phase treatment. It can be clearly seen that the secondary nanoplates are uniformly assembled on the whole side surface of the primary ZnO-NRs and the nanoplates are exible with a lateral size of ca. 280 nm. From the compositional information of the point-scan EDX in Fig. 2c, an S element is found to be present along with Zn and O, providing powerful evidence that the S element is successfully incorporated into the ZnO-NRs surface. Further elemental analysis by combustion for the product indicates that the material includes C 6.09%, H 1.62%, N 2.42%, and S 6.04%. Accordingly, the atomic ratio of C, H, N and S was 6.0 : 19.2 : 2.0 : 2.2, demonstrating 1,6-hexanediamine was inserted into the product in the molecule form (the slightly higher content of hydrogen was due to the adsorption of water molecules in surroundings), and the possible formula of ZnS hybrid material can be determined as ZnS$(HDA)0.5. Moreover, the product was composed of about 70.7% ZnO and 29.3% ZnS
Fig. 1 XRD pattern of the as-prepared ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructure, which is obtained by detaching the sample from the zinc foil by ultrasonication in ethanol and drying it in a vacuum.
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Morphology and structural characterization of the ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure. ((a) and (b)) Low and high magnification FESEM images, (c) EDX spectrum, (d) TEM image of an individual ZnO-NRs/ZnS$(HDA)0.5-NPs, (e) magnified TEM image of a part of individual ZnO-NRs/ZnS$(HDA)0.5-NPs, showing the flexible and nearly transparent nanoplate structure, and (f) HRTEM image of the junction, which shows the epitaxial growth relationship of the ZnS nanoplate on the ZnO nanorod. Fig. 2
Fig. 3 FESEM images of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures synthesized with other sulfur sources. ((a) and (b)) The product with thioacetamide as the sulfur source, and ((c) and (d)) the product with thiosemicarbazide as the sulfur source.
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Fig. 4 XRD patterns of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures synthesized with other sulfur sources. (a) The product with thioacetamide as the sulfur source, and (b) the product with thiosemicarbazide as the sulfur source.
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Scheme 1 Schematic image of the fabrication of ZnO-NRs/ ZnS$(HDA)0.5-NPs heterostructure on a zinc substrate. (a) Small ZnS hybrid nanoplates covered on the side surfaces of needle-like ZnONRs (step 1), and more intensive and larger lateral size nanoplates generated on thinner ZnO-NRs, forming the final ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure (step 2), and (b) the proposed formation mechanism of the ZnS$(HDA)0.5 nanoplates. For clarity, hydrogen atoms are omitted in the structure of 1,6hexanediamine.
Fig. 5 FTIR spectra of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures. ((a)–(c)) Samples 1, 2, 3, respectively.
hybrid (according to the S element analysis). The detailed crystallinity and composition of as-synthesized hierarchical heteronanostructure were further examined by TEM measurements. A typical TEM image of an individual ZnO-NRs/ ZnS$(HDA)0.5-NPs heteroarchitecture in Fig. 2d further clearly reveals that exible and nearly transparent ZnS$(HDA)0.5-NPs are attached on ZnO-NR, and the backbone of ZnO-NR was about 2.8 mm long and 330 nm wide near the half height of the nanorod, which is ner than that of pure ZnO-NRs (about 380 nm wide). This indicated the formation of ZnS$(HDA)0.5NPs is from the consumption of ZnO-NRs. The magnied TEM image of a part of individual ZnO-NRs/ZnS$(HDA)0.5-NPs in Fig. 2e further presented the exible and thin nanoplate structure of ZnS$(HDA)0.5 on ZnO-NR. The HRTEM image at the edge of the nanoplate shows the continuous lattice fringes (Fig. S3†), demonstrating its single-crystal nature. The corresponding fast Fourier transform (FFT) spot diagram (Fig. S3† inset) further clearly veries the single-crystal nature of the nanoplate. The
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6 CV performances of bare GCE and the ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure of a Sample 3 modified electrode in the absence and presence of N2H4 in 0.1 M PBS of pH 7.0 at a scan rate of 50 mV s1. Fig.
HRTEM image in Fig. 2f clearly identied the great dark/bright contrast along both sides of the interface, where different crystal parts are displayed. Although some stacking faults and other crystal defects are present in the interfacial region, both the backbone and branch exhibit clear lattice fringes. One set of the planar spacing was about 0.26 nm, which corresponded to the (002) plane of hexagonal wurtzite-type ZnO. Another set of the fringes' spacing measured ca. 0.31 nm, corresponding to the (008) lattice spacing of hexagonal ZnS, indicating that the ZnS
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Fig. 7 Electrochemical impedance spectroscopy of different electrodes, which were obtained in 0.5 M KCl solution containing 5.0 mM Fe(CN)63/Fe(CN)64.
Fig. 8 CV performances of different ZnO-NRs/ZnS$(HDA)0.5-NPs and ZnO-NRs modified electrodes in the presence of 200 mM N2H4 in 0.1 M PBS of pH 7.0 at a scan rate of 50 mV s1.
Fig. 9 Typical amperometric responses of the ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure of Sample 3-modified electrode with successive addition of 50 mM N2H4 in 0.1 M PBS of pH 7.0 at 0.5 V.
nanoplate branch was indeed grown over the ZnO nanorod backbone. Upon further examination of the interface, an
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epitaxial growth relationship between the ZnO-NRs and ZnSNPs component is also suggested, that is [002]ZnOk[008]ZnS. The HRTEM image reveals the good single-crystalline nature of the ZnS nanoplate, which preserves the crystal structure characteristics and follows the crystallographic orientation of the initial ZnO nanorod. Therefore, the secondary ZnS hybrid nanoplates have a layered structure in which 1,6-hexanediamine acts as the interlayer molecules connecting ZnS layers. Investigation of the reaction conditions, such as the reaction time, the concentrations of 1,6-hexanediamine and Na2S, and other sulfur resources on the microstructures of secondary ZnS hybrid nanoplates, have been carefully investigated. The morphology evolution of the intermediates involved in the formation caught at 10 min, 30 min and 60 min showed some tiny nanoplates covered on the nanorods aer 10 min (Fig. S4a†), and with time prolonged to 30 min, the product was larger lateral size nanoplates covered on the nanorods (Fig. S4b†). At 60 min, the nanoplates with much larger lateral size and more intensive structure on ZnO-NRs are observed (Fig. S4c†). And at 8 h, large exible nanoplate-like structure uniformly covered on ZnO-NRs is obtained (Fig. 2a and b). As the reaction time was further increased to 24 h, the morphology of the heterostructure is barely unchanged (Fig. S4d†). Timeresolved experiments revealed that the suldation process began at the surface of the ZnO-NRs with some small nanoplates formed rst, which gradually expanded, and generated more intensive and larger lateral size nanoplates on ner ZnONRs. When the growth achieved dynamic equilibrium at a certain reaction stage, the microstructure of the heterostructure remained basically unchanged with further aging. The further investigations on the concentration effects of 1,6-hexanediamine and Na2S showed that as the amount of 1,6hexanediamine increased to 4 mL while keeping other parameters constant, hierarchical heterostructures with more ZnS hybrid nanoplates on ZnO-NRs were obtained (Fig. S5a†), demonstrating that a moderate increase of 1,6-hexanediamine will favor the conversion of ZnO-NRs into ZnS hybrid nanoplates. But, when further increased to 6 mL, the product was obviously less regular (Fig. S5b†), which may be due to the overhigh concentration of 1,6-hexanediamine promoting the rapid formation of ZnS hybrid nanoplates branches at the rapid dissolution of ZnO-NRs backbones. In addition, when the Na2S concentration was doubled, more intensive ZnS hybrid nanoplates were covered on the ZnO-NRs (Fig. S5c†), indicating that the more sulfurous the source is, the more transformations the ZnS hybrid nanoplates on ZnO-NRs will have. These results suggest that the elegant ZnO-NRs/ZnS-HDA-NPs hierarchical hybrid heterostructures can be obtained at suitable concentrations of 1,6-hexanediamin and Na2S. It is worth noting that the strategy for obtaining hierarchical ZnS-HDA-NPs on ZnO-NRs by 1,6-hexanediamin-assisted synthesis is not limited to Na2S as the sulfur source but also can be extended to other sulfur sources, such as thioacetamide and thiosemicarbazide. When using thioacetamide as the sulfur source, the ZnS-HDA-NPs covering the ZnO-NRs surface were thicker (Fig. 3a and b). The corresponding TEM image (Fig. S6a†) further revealed the hierarchical nanoplates covering
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the nanorod, and the HRTEM image from the junction (Fig. S6b†) also displayed two types of clear lattice fringes, which can be indexed to the (002) and (008) lattice spacing of single crystalline hexagonal ZnO and ZnS, respectively. While using thiosemicarbazide as the sulfur source, the nanoplates grown on the side surfaces of ZnO-NRs become longer and ner and form an elegant ower at the top of the ZnO-NRs (Fig. 3c and d). The TEM images (Fig. S6c and d†) and HRTEM images (Fig. S6e and f†) further showed the hierarchical nanostructure and single-crystalline nature of the ZnS-NPs. The corresponding XRD patterns shown in Fig. 4 were similar to Sample 1. In addition, the small-angle diffraction peaks shi slightly towards lower angles for Sample 3, demonstrating that the d-spacing of the planes increased. The elemental analysis showed the products include 9.77% C, 2.41% H, 4.04% N and 9.46% S for Sample 2, and 10.91% C, 2.78% H, 4.22% N and 9.80% S for Sample 3. Further analysis reveals the atomic ratio of C, H, N and S was 6.0 : 17.8 : 2.1 : 2.2 for Sample 2, while it was 6.0 : 18.4 : 2.0 : 2.0 for Sample 3. The possible formula of the ZnS hybrid materials can still be determined as ZnS$(HDA)0.5. Meanwhile, the products were composed of about 54.1% ZnO and 45.9% ZnS hybrid for Sample 2 and 52.4% ZnO and 47.6% ZnS hybrid for Sample 3 (according to the S element analysis). The higher elemental contents demonstrated more ZnS hybrid nanoplates were formed from the consumption of ZnO-NRs, which is consistent with the results of FESEM and TEM. The further investigations on the concentrations of 1,6-hexanediamine and corresponding sulfur sources give us more opportunities to tune the microstructure of ZnS-HDA-NPs covered on ZnO-NRs (Fig. S7†). The experimental results also suggest that the temperature is another important parameter affecting the microstructures of secondary ZnS-HDA-NPs (Fig. S8†). To elucidate the effect of the zinc substrate on the formation of the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructure, an experiment using ZnO-NRs detached by ultrasonication from the zinc substrate was performed in the second liquid-phase conversion process with other parameters kept constant. Aer the reaction, ZnO-NRs/ZnS$(HDA)0.5-NPs were still generated, but the product was poorly dened (Fig. S9†). The result implied that the zinc substrate plays an important role for obtaining welldened ZnO-NRs/ZnS$(HDA)0.5-NPs. In our case, the aligned needle-like ZnO-NRs stood on a zinc substrate possess a high distribution density and a wide separation between the neighboring nanorods, supplying a good opportunity for the uniform conversion of ZnS$(HDA)0.5-NPs on the large exposed surfaces of ZnO-NRs via the second liquid-phase chemical conversion process, whose similar role has been demonstrated on ZnONRs/Ag-NPs heterostructures.36 In order to clarify the effect of 1,6-hexanediamine on realizing the formation of hierarchical ZnS$(HDA)0.5-NPs branches, an experiment with NH3$H2O instead was performed in the second conversion step. Aer reaction, only tiny ZnS nanoparticles covered on the ZnO-NRs were obtained (Fig. S10†). The results demonstrated that the 1,6-hexanediamine plays a particular role on the formation of the ZnS$(HDA)0.5-NPs branches.
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In this approach, the formation of ZnS$(HDA)0.5-NPs on ZnO-NRs is due to the suldation conversion in the second liquid-phase growth process through the reaction of ZnO-NRs with the sulfur source in the presence of 1,6-hexanediamine. However, what role does 1,6-hexanediamine play in terms of controlling crystal growth? Being a kind of water-soluble alkylamine with a bidentate ligand, two lone pairs of electrons associated with the two nitrogen atoms interact with cationic zinc ions. At the beginning of the suldation reaction, S2 anions released from the decomposition of the sulfur source at elevated temperature react with the Zn2+ slowly dissolved from the surface of ZnO-NRs, producing ZnS nanoparticles around the ZnO-NRs. Further, 1,6-hexanediamine preferentially bonded on the ZnS surface perpendicular to the (001) facet through surface ions, and thus passivated the surface atoms, resulting in the ZnS crystal preferring to grow along [001] and [010] directions into 2D nanostructure. At the same time, 1,6hexanediamine molecules are incorporated into neighboring ZnS layers by coordination. As the suldation time increased, the nanoplates further grow and generate uniform and dense ZnS$(HDA)0.5 nanoplates’ branches. Thus, ZnO-NRs/ ZnS$(HDA)0.5-NPs hierarchical heterostructure gradually form. Therefore, it is the preferential binding of 1,6-hexanediamine on specic facets of ZnS that makes the thickening rate much lower than the lateral growth rate; meanwhile, 1,6-hexanediamine molecules were inserted into the interlayers of ZnS by coordination of N atoms with neighboring layers in a vertically aligned mode, leading to the formation of 2D ZnS$(HDA)0.5-NPs covered on ZnO-NRs. The Fourier transform infrared (FTIR) spectra of the heterostructure samples conrmed the coordination of 1,6-hexanediamine in the products (Fig. 5). The sharp absorption peaks observed at 3242 cm1 and 3125 cm1 were the stretching vibration of the amino group, which are shied to the lower frequencies (usually at 3500–3300 cm1) due to the binding of the amino group in the heterostructure. The absorption peaks observed at 2930 cm1 and 2844 cm1 were associated with the stretching vibration of a C–H bond in methylene. Meanwhile, the band at 1470 cm1 and 1357 cm1 can be indexed to the bending vibrational mode of C–H bond in methylene. The absorption peaks observed at 1615 cm1 and 1515 cm1 were the bending vibration of N–H bond in amino group. The bands at 1072 cm1 and 996 cm1 are attributed to the stretching vibration of C–N bond. Therefore, the FTIR spectra conrmed the existence of 1,6hexanediamine in the heterostructures, which supported our mechanism from another point of view. The microstructure variation of ZnS$(HDA)0.5-NPs in the heterostructure with changes of the sulfur source may be associated with the difference of the concentration of S2 decomposed from the sulfur source during the reaction process at the designed temperature and the assisted coordinated role of the sulfur source. Of course, it still needs more detailed and systematic work to provide evidence to make clear the precise functions of 1,6-hexanediamine and the synergistic effect of the sulfur source on the shape control of ZnS$(HDA)0.5-NPs branches. Based on the experimental results, the formation mechanism of ZnO-NRs/ ZnS$(HDA)0.5-NPs is schematically shown in Scheme 1.
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The synthesized heterostructure products have been investigated for electrochemical catalysis of hydrazine to evaluate their chemical sensing properties. Fig. 6 shows the cyclic voltammograms of the bare GCE and the ZnO-NRs/ZnS$(HDA)0.5NPs hierarchical heterostructure of a Sample 3 modied electrode in the absence or presence of hydrazine. Comparing with the bare GCE, a weak redox peak appeared on Sample 3/Nf/GCE, which proved that the prepared ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heteroarchitecture was successfully modied onto the GCE. When the hydrazine was added, the Sample 3/Nf/GCE electrode exhibited an intense electrocatalytic activity for hydrazine oxidation. The catalytic oxidation is sharp and occurs at about 0.2 V, reecting a faster electron-transfer reaction owing to the high catalytic effect of the special heteronanostructure. However, the oxidation of hydrazine requires very high positive overpotentials at bare GCE, leading to a poorly dened anodic wave characteristic of very slow electrode kinetics. In order to study the electrocatalytic activity of different ZnONRs/ZnS$(HDA)0.5-NPs hierarchical heterostructures, the electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) spectra were obtained for the samples. Electrochemical impedance spectroscopy (EIS) can provide useful information on the impedance changes of the electrode surface during the fabrication process. The Nyquist plot of the EIS includes a semicircular portion and a linear portion. The semicircular portion at higher frequencies corresponds to the electron-transfer limited process and its diameter is equal to the electron transfer resistance (Ret), which controls the electron transfer kinetics of the redox probe at the electrode interface. Meanwhile, the linear part at lower frequencies corresponds to the diffusion process.39 Fig. 7 displays the Nyquist plots of the EIS of different electrodes. Aer three different ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructures (Samples 1 to 3) and ZnO-NRs were attached onto the GCE, the semicircle diameter of EIS, Ret, is different for the different electrodes. The EIS was in a sequence with Ret ZnO > Ret Sample 2 > Ret Sample 1 > Ret Sample 3, which meant that the electron transfer ability is Sample 3 > Sample 1 > Sample 2 > ZnO. Obviously, the Ret ZnO is much larger than that of three kinds of ZnO-NRs/ ZnS$(HDA)0.5-NPs heterostructures, indicating the conductivity of the heterostructure is much better than ZnO-NRs. Herein, the increased conductivity of ZnO-NRs/ZnS$(HDA)0.5-NPs may be attributed to the intimate mixing at the interface of the ZnO-NRs and ZnS$(HDA)0.5-NPs, which forms excess Zni donors due to bond rearrangement by the electronegativity difference between S and O.18 Because Zni is known to act as shallow donors in ZnO,40 the ZnO-NRs/ZnS$(HDA)0.5-NPs heterostructure becomes more conducting. The ZnO-NRs/ZnS nanoparticles core–shell nanostructure synthesized in the presence of NH3$H2O appeared as a partially-dissolved ZnO core/ZnS shell nanostructure (Fig. S10c†), the Ret of the ZnO-NRs/ZnS nanoparticles is even larger than that of ZnO-NRs (Fig. S11†). Meanwhile, the formation of the ZnO-NRs/ ZnS$(HDA)0.5-NPs heterostructure obviously increases the surface area of the product, which also makes the electrochemical probe arrive at the electrode's surface easily. The electrochemical responses of hydrazine on three different ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructures
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and ZnO-NRs were further investigated (Fig. 8). All the three hierarchical heterostructures have an improving effect on the response of hydrazine compared with ZnO-NRs, and the largest increment of the peak current and most well-dened peak with the same concentration of hydrazine appeared on Sample 3. The superior electrochemical sensing property of the heterostructure can be explained by the improved conductance and increased surface area. The results also showed the microstructure of ZnONRs/ZnS$(HDA)0.5-NPs had a noticeable inuence on their electrochemical catalytic properties, which can be mainly attributed to the variations in surface area. The sample with a larger surface area enables more reactive-sites for electrocatalysis, improving the corresponding sensing performance. The differences in the CV results were consistent with the variations of EIS. Fig. 9 shows a typical amperometric response of the hierarchical heterostructure of Sample 3 to the successive addition of hydrazine (50–1000 mM) into continuously stirred 0.1 M PBS (pH ¼ 7.0) at an applied potential of 0.5 V. The amperometric sensor exhibited a rapid and sensitive response to the change of hydrazine concentration with the oxidation current obviously increased upon successive addition of hydrazine. An inset in Fig. 9 shows the corresponding calibration curve of the fabricated amperometric hydrazine sensor. The results indicate that the ZnO-NRs/ZnS$(HDA)0.5-NPs hierarchical heterostructuresmodied electrodes have a potential application for N2H4 chemosensor.
Conclusion In summary, we describe a simple liquid-phase chemical conversion reaction with ZnO nanorods on a zinc substrate to produce ZnO nanorods/ZnS$(HDA)0.5 hierarchical heterostructures with a tunable number density and thickness of the secondary ZnS$(HDA)0.5 nanoplate-like structure. In this selfassembly process, the preferential binding of 1,6-hexanediamine on specic facets of ZnS plays a particular role for the formation of the hybrid ZnS$(HDA)0.5 nanoplates’ branches and accounting for the insertion of 1,6-hexanediamine molecules into interlayers of ZnS. The as-synthesized hierarchical heteroarchitectures exhibited superior electrochemical catalytic properties for N2H4, which can be attributed to the better conductivity and larger surface area of the heterostructures. It is undoubtedly only one of their properties, leaving the expanded chemical functionality and physical properties to be explored. Meanwhile, this simple and efficient two-step liquid-phase strategy with preferential binding of water-soluble alkylamine for the orientation-regulated hierarchical hybrid nanoheterostructures is expected to allow the fabrication of other complex semiconductor hybrid heterostructures with an intrinsic large lattice mismatch, thus opening up new avenues for bringing on series of unprecedented excellent properties.
Acknowledgements This work was nancially supported by the Natural Science Foundation of China (21201007 and 21271009), the Program for New Century Excellent Talents in University (NCET 11-0888),
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and Natural Science (11040606M32).
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