Ag nanowires as precursors to synthesize Ag–ZnO nanostructured brushes

Abstract

Ag–ZnO nanostructured brushes (NBs) were synthesized by the precipitation method. From this study, it was observed that a key parameter to grow these unidirectional structures is the seeding through Ag nanowires. SEM analysis showed that the Ag nanowires serve as templates in the formation of Ag–ZnO NBs with a one-directional shape. The absence of Ag nanowires leads to the radial growth of ZnO with a star-shaped morphology. The obtained Ag–ZnO unidirectional structures have potential catalytic, bactericide, and conducting applications, and may also find application as a nanorotor.

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Introduction

Nanorods (NRs), nanotubes, nanowires (NWs) and nanoparticles (NPs) arrays have attracted great interest due to their unique properties and wide range of potential applications in a number of nanotechnologies. The synthesis of metal nanostructures today is one of the most intensively researched fields of preparative chemistry.^1–3 Due to their small sizes and large specific surface areas, nanostructures exhibit novel properties, which may significantly differ from those of the bulk materials.^4–6 In particular, one-dimensional (1D) nanomaterials have attracted considerable attention for solar cell applications, next-generation field emitters, UV-photodetectors, for electronic interfaces with biological systems, gas sensors, and for catalysis.^7–14 The growth of aligned NRs and NWs is important for applications such as lasers, light-emitting diodes and field effect transistors. ZnO is one of the key nanomaterials for the development of nanotechnology. Recently, a piezo-electric nanogenerator (NG) that converts mechanical energy into electric energy has been developed, based on the coupling of a piezoelectric with the semiconducting properties of ZnO nanostructures.^15,16 The aligned growth of NRs and NWs can be achieved through the use of substrates and catalyst particles or seeds. The large-scale perfect vertical alignment of ZnO nanowires was first demonstrated over a (112̄0) crystal surface of an oriented single-crystal substrate of aluminum oxide (sapphire).¹⁷ Gold nanoparticles have been used as seeds to grow ZnO on a Si wafer.^18,19 In this system the growth is initiated and guided by the Au particle and the epitaxial relationship between ZnO and the Si wafer leads to the alignment of the ZnO nanowires.¹⁹ The laterally aligned NWs were prepared using different materials to activate or inhibit the growth of the nanowires.^20,21

In this study, we report the controlled growth of Ag–ZnO NBs by the precipitation method at room temperature. Ag-NWs were used as the precursors to grow the ZnO along these nanostructures. The final shape of this nanostructured material is a brush-like unidirectional structure. This new material can be used as a catalyst for steam reforming or for photocatalysis, where silver participates as the active phase.^12–14 However, the Ag–ZnO NBs can also be applied as a conducting material, for bactericidal applications or as a nanorotor.

Experimental

The Ag nanowires were synthesized through a PVP (polyvinylpyrrolidone)-assisted reaction in ethylene glycol.²² An aqueous solution of zinc nitrate (Zn(NO₃)₂·6H₂O) and the Ag nanowires were mixed under constant stirring, and the molar ratio of Ag/ZnO was 0.075. Then, a solution of ammonium hydroxide (NH₄OH) at 28% was added dropwise to complete the precipitation at 19 °C. The solution was stirred for 30 min and the precipitated mixture was aged for 24 h. The residual liquid was removed by decanting. The precipitate was heated at 50 °C for 24 h. The resulting material was then calcined at 500 °C for 5 h under static air and reduced in a H₂ flow at the same temperature for 1 h. Finally, the sample was cooled down slowly to room temperature (R.T.).

Morphological characterization was performed in a low vacuum scanning electron microscope (LVSEM) Jeol JSM-6610-LV at 20 kV, equipped with an energy dispersive X-ray spectroscope (EDX) INCAX-act-Oxford. Powder X-ray diffraction (XRD) patterns were recorded using a Siemens D-5000 diffractometer, from 2θ = 27–67°, at 40 kV, using Cu Kα (λ = 0.15406 nm), step = 0.026°, and time = 3 s. The Rietveld refinement method and TOPAS program were used to determine the crystallite size and to quantify the phases of the Ag–ZnO NBs from the XRD diffraction patterns. For the TEM and HREM experiments, the sample was ground in an agate mortar and suspended in isopropanol. After ultrasonic dispersion, a droplet was deposited over a carbon-coated copper TEM grid. Microstructural analysis was performed in a TEM Jeol JEM 2010, with a resolution of 0.19 nm fitted with an energy dispersive X-ray spectrometer (Noran, model Voyager 4.2.3).

Results and discussion

A typical SEM image of the ZnO structures with a star-shape is shown in Fig. 1a. This sample was prepared without Ag NWs. It can be observed by scanning electron microscopy (SEM) that these structures grow in a radial form with a star-like shape. The peak-to-peak distance of one of the axis of these ZnO star-shaped structures is close to 3 μm. In all cases, the “cones of the star” were formed by bunches of needles distributed randomly. This type of morphology was obtained previously in different studies.^18,23–25 In an earlier study,¹⁶ it was suggested that the concentrations of [OH⁻] and [Zn²⁺] in the solution are the most important parameters for obtaining these type of structures, and they affect the nucleation and growth of the ZnO structures. A low concentration of [OH⁻] and [Zn²⁺] during the synthesis prevents the star-shaped structures growing in a preferential direction in order to obtain unidirectional structures. In this research, Ag nanowires, shown in Fig. 1b, were used as templates to obtain the Ag–ZnO unidirectional nanostructures. These Ag nanowires have average dimensions of 80 nm in diameter and 15 micrometers in length.²² The growth direction of the Ag-NWs was along the [111̄] axis. The analysis of the transversal section of those nanowires was found to have a pentagonal symmetry.

Fig. 1 SEM images of the (a) ZnO structures with a star-shaped morphology, (b) Ag nanowires, (c) Ag–ZnO NBs and (d) energy-dispersive X-ray spectroscopy of (c) Ag – 4.21 at%.

When the Ag NWs and the precursor of the ZnO were added together, the Ag–ZnO NBs-like morphology was obtained (Fig. 1c). It is clear that the Ag-NWs are necessary to obtain the Ag–ZnO nanostructured brush-like morphology, because without them, only individual ZnO nanostars were grown. The energy dispersive spectra of the sample obtained from the SEM-EDS analysis (Fig. 1d) clearly shows that the Ag–ZnO NBs are composed of O, Zn and Ag with an atomic ratio of Ag : Zn of ∼4 : 32.

Changes on the ZnO morphology due to the introduction of silver into the system were reported by Zhang et al.²⁶ They observed that silver influences the growth of the ZnO structures and concluded that it could be used specifically to control the final morphology of the ZnO to achieve a specific type of nanostructure. In our case, we observed a similar behavior, where the ZnO structures grew along the silver NWs as brushes with a star-like morphology and a five-fold symmetry (Fig. 1c). The distance from cone to cone in the transversal growth axis of these Ag–ZnO unidirectional NBs was close to 1.3 μm, which is less than that of the star-like ZnO structures prepared without silver. Fig. 2 shows the XRD pattern of the ZnO and Ag–ZnO NBs samples after the thermal treatments (calcination and reduction). The X-ray diffraction pattern of the samples was indexed as a hexagonal wurtzite structure of ZnO with the lattice parameters a = b = 0.3241 nm and c = 0.5187 nm (JCPDS card no. 073-8765). These diffraction peaks at the scattering angles 2θ were indexed and are shown in Fig. 2. In addition, diffraction peaks of metallic Ag were present in the Ag–ZnO NB samples. No extra peaks related to an intermetallic Ag–Zn compound were observed. These results indicate that the Ag phase remains within the unidirectional structure, even when its original shape is no longer observed. The quantitative phase composition and particle size grain of the Ag–ZnO NBs were determined using the Rietveld refinement procedure (ESI†). The obtained R factors were R_wp = 5.393, R_exp = 4.846 and χ² = 1.11. The mean crystallite size of the ZnO of the Ag–ZnO NBs was ca. 8.20 nm. The corresponding quantitative fraction of the Ag and ZnO phases were 10 and 90 wt%, respectively. These values are close to the nominal content in the sample.

Fig. 2 XRD patterns of ZnO nanostars and Ag–ZnO NBs. The dashed line corresponds to the calculated XRD pattern by the Rietveld method.

The typical TEM images of the obtained samples in Fig. 3 show the overall morphology of the Ag–ZnO composites. They reveal that the Ag–ZnO NBs grew unidirectionally with a nanostructured brush-like morphology and with cone-like growths on the tips of the structure (Fig. 3A and B). EDS analyses of these cones indicated the following elemental composition: Zn (60.96 at%), O (38.43 at%) and Ag (0.61 at%). A transversal view of the Ag–ZnO NBs is presented in Fig. 3C and D, where the five-fold symmetry is evidenced. It is important to mention that along the unidirectional structure, it is possible to observe a clear line. This phenomenon was observed previously during the preparation of Ag–CeO₂ nanotubes,^13,14 in which Ag NWs were used to synthesize these kinds of unidirectional structures.

Fig. 3 Ag–ZnO NBs TEM images: (A and B) longitudinal view and (C and D) transversal view.

Fig. 4 shows the cross-sectional images of the Ag–ZnO NBs, where it can be observed that the brushes have a star-like morphology (Fig. 4A and B). The width of the star, i.e., cone to cone, is from 1050 to 1250 nm (Fig. 4C and D). In the micrographs of the Ag–ZnO NB slices, as shown in Fig. 4A, it can be observed that a few pentagonal Ag-NWs can remain in the center of the NBs, which is associated with the highly contrasted structure in the middle of the slice. However, when the Ag NWs were removed (the clear line in Fig. 3A and B), the presence of a pentagonal central hole, with dimensions between 200 and 350 nm, is also evidenced, which might be due to the previous existence of the pentagonal NWs²² (Fig. 4B). The arrows in Fig. 4C and D show that Ag NW is absent in some parts of the Ag–ZnO NBs, which could be the origin of the clear line observed in Fig. 3. However, in others zones of the material, the Ag NW still remains within the center of the structure. Fig. 4C shows a central black line 120 nm wide and a grey fringe of approximately 110 nm width. According to the EDS analysis, this black line was mainly silver (76.38 at%), with a low concentration of Zn (19.74 at%) and O (3.88 at%), indicating that the Ag-NW is still present. However, in the grey fringe, Zn (84.83 at%) and O (13.58 at%) were the main elements detected, along with a small amount of silver (1.59 at%). According to our observations, we suggest that the Ag-NW might have diffused through the ZnO structure during the thermal treatments by following a Kirkendall mechanism. In a first stage, the Ag-NWs were used as templates to grow epitaxially a ZnO phase around the Ag-NW (Scheme 1). This phenomenon is similar to that reported by Yong Ding et al.,²⁷ where ZnO nanostructures with an asymmetric growth were formed and they named them “combs”. In our case, we obtained a well-defined symmetric structure.

Fig. 4 Cross-sectional TEM images of the Ag–ZnO NBs. Star-like nanostructures (A and B) and Ag–ZnO NBs (C and D).

Scheme 1 Grow mechanism of the Ag–ZnO NBs.

Fig. 4D shows the cross-sectional image of a Ag–Zn NB, where the Ag NW was removed during the synthesis and thermal treatments. The arrow indicates a less contrasted zone of 83 nm in size. It is suggested that this zone was originally occupied by the Ag-NW. This ZnO structure grew close to 110 nm radially around the Ag-NW, and then continued growing with a star-like morphology, which then properly formed the brushes. The Ag NW diffused to the ZnO structure through a Kirkendall effect (Scheme 1). This effect was observed elsewhere^13,14 in Ag NWs covered by CeO₂, where nanotubes of CeO₂ were grown over Ag NWs and the Ag diffused into the CeO₂ nanotubes during the synthesis process.

A HREM image from the tip of one cone in a Ag–ZnO NB is shown in Fig. 5. In this figure, it can observe that the structure is formed by ZnO nanocrystals with an average size of 5 nm. According to the lattice spacing measured, these nanocrystals correspond to a hexagonal phase with parameters a = b = 0.3241 nm and c = 0.5187 nm, as previously identified. The inset in Fig. 5 shows a magnification of one of these nanocrystals, where the incident electron beam is along the [0001] zone axis and the FFT image allowed the identification of the crystallographic planes (1̄010) and (011̄0) on it. These ZnO nanocrystals, which grew along the 〈0002〉 direction, formed the “bristles”. Based on these results it was determined that the brushes grew in that same direction. The dark field image taken with the diffracted (0002) plane of the ZnO, showed that the Ag/ZnO NBs are formed by nanocrystals (ESI†).

Fig. 5 HRTEM image from one of the “cones” shown in Fig. 4b. Inset shows the FFT and HRTEM of the black frame on the image.

Conclusions

A novel approach to produce unidirectional Ag–ZnO nanostructured brushes has been presented, based on the use of unidimensional Ag-NW templates. The absence of Ag nanowires leads to the radial growth of ZnO with a star-like shape. For the preparation of the Ag–ZnO NBs, we suggest that in a first stage an epitaxial growth of the ZnO around the Ag-NWs takes place and then it continues growing until a structure with a star-like morphology is obtained, which properly conformed the brushes morphology. During thermal treatments, the Ag-NWs diffuse onto the ZnO structure through a Kirkendall effect, and in some of these new NB structures, the Ag-NWs are no longer observed, whereas in others, the Ag-NWs still remain. This could be attributed to the kinetics of the reaction.

Acknowledgements

This research is supported by the ININ (grant no. CA-409) and CONACyT (grant CB-169682). Thanks to J. Quezada and A. Gutiérrez Martínez for technical support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra04261e

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