Herein, we demonstrate that uniform Ag nanoparticles could be directionally grafted on the tip of ZnO nanowire arrays by a simple photo-reduction method. Furthermore, the structure, position, and amount of Ag nanoparticles supported on ZnO nanowire arrays could be further rationally tailored by changing the reaction parameters such as the category, concentration of reagents, and annealing temperature. Moreover, their photoelectrochemical performances under both UV-vis and monochromatic light irradiation have been explored. Interestingly, the photocurrent density of Ag–ZnO heterostructures could reach up to 2.40 mA cm
2 , which is much higher than that of pure ZnO nanowire arrays. It has been proposed that the formation of ZnO nanowire arrays tip-grafted with Ag nanoparticles could promote the effective separation and directional transfer of photoexcited electron–hole pairs, and thus enhance the photoconversion properties.

1 Introduction

With a wide direct band gap (3.4 eV) and large exciton binding energy (60 meV), ZnO has attracted intense interest due to its great potential photonic applications in the visible and the near-UV wavelength regions. Until now, ZnO nanowire arrays have been one of the most intensively studied nanomaterials which can be widely used for energy harvesting, sensing, optoelectronic, and photocatalytic applications.1–6 More specically, ZnO nanowire arrays with a high aspect ratio can offer not only a large surface area for charge transfer and a short diffusion length for minority carriers, but also a long pathway for light absorption. However, the relatively low conversion efficiency and high recombination rate of photoexcited electron–hole pairs greatly limited their practical applications. To settle these fundamental issues, much effort has been focused on the hetero-coupling with metallic nanomaterials to ZnO nanowire arrays for enhancing their optoelectric as well as photocatalytic performances. More recently, silver especially has attracted intense interest because of its efficient surface plasmon resonance in the visible light region. Moreover, the metallic Ag could effectively facilitate efficient charge separation, and thus suppress the recombination of photoexcited electron–hole pairs and improve the photoelectrochemical performances.

However, the current Ag–ZnO heterostructures7–11 were generally obtained by randomly and uncontrollably loading Ag nanoparticles onto the surfaces of ZnO nanowire arrays. Accordingly, their stochastic combination is not suitable for studying the properties related to the anisotropy of ZnO crystals in detail. Moreover, the formed Ag nanoparticles inevitably destroy the compact architectures and harshen the smooth surfaces of ZnO nanowire arrays, which greatly reduced their chemical and physical properties and the potential for practical applications in fabricating nanodevices. Although the facetselective epitaxial growth of the ZnO nanorods on Ag nanocrystals has been reported,12 their preparation procedure was realized via a complex and time-consuming seed mediated method and the as-prepared heterostructure was dispersed in water, which is not available for repeated use in many occasions. Therefore, it is highly desirable to develop a simple and effective strategy for site-selective growth of Ag nanoparticles on the ZnO nanowire arrays while maintaining the original architectures and structures for photoelectrochemical application.

Herein, we demonstrate a facile and reliable synthetic strategy to directionally gra Ag nanoparticles on the tips of ZnO nanowire arrays through a simple photo-deposition reduction method. The amount, position, and structure of Ag nanoparticles can be rationally tailored by adjusting the reaction parameters. On the basis of the above results, a possible growth mechanism of the Ag nanoparticle tip-graed ZnO nanowire arrays was proposed. Furthermore, their potential applications as the photoelectrode in photoelectrochemistry have been explored. These Ag–ZnO nanowire array heterostructures exhibited a much better photoelectrochemical performance than pure ZnO nanowire arrays. More specically, the as-prepared hetero-products have some major merits: (1) most Ag nanoparticles were deposited on the tip of the ZnO nanowire, which made the photoelectrochemical reaction more efficient because most reactions were carried out on the top-surface of the nanowires; (2) the interaction between silver and ZnO promotes the effective separation of electron–hole pairs and inhibits their recombination because of the low Fermi level of metallic silver; (3) the heterostructure is more stable than pure ZnO nanowire arrays due to the gra of Ag nanoparticles on the metastable (002) facet of ZnO.1

2 Experimental

2.1Material

All the chemicals were of analytical grade and used as received without any further purication, unless otherwise stated. All chemicals were purchased from Sinopharm Group Chemical Reagent Co. Ltd., China. Deionized water with a resistivity of 18.25 MU cm was used in all reactions. Indium-tin oxide (ITO)- coated glasses were purchased from Zhuhai Kaivo Electronic Components Co. Ltd., China.

2.2 Preparation of the Ag–ZnO nanowire arrays As illustrated in Scheme 1, the nal products were prepared through two steps: the fabrication of the ZnO nanowire arrays and the synthesis of the modied ZnO nanowire arrays with Ag nanoparticles selectively grated on the tips of the nanowires. Preparation of the ZnO nanowire arrays. The ZnO nanowire arrays were synthesized via a modied hydrothermal method, which has been reported by other groups.14,15 Firstly, zinc acetate dehydrate [Zn(CH3COO)2$2H2O] was added in ethylene glycol monomethyl ether [CH3O–CH2–CH2OH] under continuous stirring. Then an optimized amount of diethanolamine (DEA) was dropped into the above solution to enhance the dissolution of zinc salts. The molar ratio of zinc acetate to DEA was kept at 1.0 and the concentration of zinc acetate was 1.2 M. The above mixed solution was stirred at 60 C for 30 min. The as-prepared 1.2 M Zn2+ solution was stabilized at room temperature for another 2 days. Then the sol was spin-coated on an ITO-coated glass substrate several times at a spinning rate of 2000 rpm using a vacuum spin coater (VTC-100). After each coating process, the substrate needed to be heated in an oven at 200 C for 30 min. Afer the repeated spin-coating procedure, the as-prepared thin film was calcined in a muffle furnace in air at 350 C for 30 min to obtain the ZnO seed layer. Secondly, the coated substrate with the coating side upside-down was immersed in 0.04 M zinc nitrate (Zn(NO3)2 6H2O) solution containing equal amounts of hexamethylene tetraamine (HMT) contained in a Teon liner stainless-steel autoclave to grow ZnO nanowire arrays at 95 C for 6 h. The achieved product was then removed from the solution and rinsed with deionized water and absolute ethyl alcohol several times followed by a drying step in an oven at 60 C

Preparation of Ag–ZnO heteronanostructure. The Ag nanoparticle modified ZnO nanowire arrays were prepared by a simple photo-reduction method. Silver nitrate (AgNO3) was dissolved in deionized water containing 20% absolute ethyl alcohol to form AgNO3 precursor solution with a concentration of 0.002 M. Then the ZnO nanowire arrays were immersed in the above solution, and illuminated by a 300 W Xe lamp (HSX-F/UV 300) for 2 minutes to reduce the Ag+ adsorbed on the surface of the film to Ag nanoparticles. Then the as-prepared heterostructures were rinsed with deionized water and absolute ethyl alcohol several times to remove the residual Ag+ and dried at 60 C in air.

The morphology and elemental distributions of the products are characterized using a field emission scanning electron microscope (FESEM, JSM-6701F, JEOL) employing an accelerating voltage of 5.00 kV with an energy dispersive spectrometer (EDS). X-ray diffraction analysis (XRD, Rigaku RINT-2000) using Cu Ka radiation at 40 keV and 40 mA was employed to identify the crystalline structure of our as-prepared products. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB250xi photoelectron spectrometer with X-ray monochromatisation as the excitation source to analyze samples' elemental composition. The UV-vis absorption spectra were recorded with a spectrophotometer (UV-2550)

The photoelectrochemical properties of bare ZnO nanowire arrays and the Ag–ZnO heterojunctions were measured with a typical 3-electrode system, in which a piece of Pt foil (3  2 cm) and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. And the as-prepared ZnO nanowire array and Ag–ZnO nanowire array photoanodes were employed to be the working electrodes with a surface area of 2 cm2 , respectively. The whole photoelectrochemical tests were carried out in an electrolyte medium containing 0.25 M Na2S, and 0.35 M Na2SO3, and the data were measured using an elec-trochemical workstation (CHI660D). A 300 W Xe lamp (HSX-F/UV 300) was used as the source of illumination with its full power irradiation fixed at 200 mW cm
2 with the assistance of apower meter (CEL-NP2000). Linear sweep voltammograms were measured under a bias voltage between 0 V and +0.6 V (vs. SCE) with a scan rate of 0.1 V s
1 . Amperometric I–t curves were tested at a bias voltage of 0 V and +0.2 V (vs. SCE), respectively. The optoelectrical properties of the as-prepared pure ZnO nanowire array and the Ag nanoparticle tip-grafted ZnO nanowire array under the illumination of various monochromatic light were also achieved via an automatic monochromator (7ISWS, purchased from Beijing 7-Star Optical Instruments Co., Ltd, China) and the above relevant electrochemical instruments.

3 Results and discussion

Fig. 1A and B show the typical scanning electron microscopy (SEM) images of the as-prepared ZnO nanowire arrays by a modified hydrothermal method. It can be clearly seen that the uniform ZnO nanowires are vertically aligned into well-dened arrays and have a high surface density (ca. 109 wires cm
2 ). The cross-sectional SEM image (inset in Fig. 1A) reveals that the assynthesized ZnO nanowires have lengths of ca. 1 mm and their diameters range from 100–200 nm. However, when ZnO nanowire arrays were dipped into an aqueous AgNO3 solution containing 20% absolute ethyl alcohol under illumination treatment, the graft growth between ZnO nanowires and Ag nanoparticles occurred immediately. As shown in Fig. 1C and D, uniform Ag nanoparticles have been successfully graed on the top regions of ZnO nanowires after a 2 min illumination. Interestingly, the corresponding cross-sectional SEM image clearly reveals that except for the tips of ZnO nanowires, no evident Ag nanoparticles have been observed on other areas, clearly indicating that the site-selective growth of Ag nanoparticles on the tips of ZnO nanowires has been achieved by this simple photoreduction method. Moreover, the corresponding TEM images (see Fig. S1 and S2, ESI†) further illustrate this unique Ag–ZnO heterostructures, which is highly consistent with the SEM tests. Furthermore, their compositions and crystalline structure have been systematically investigated by X-ray diffraction analysis (XRD), and the results are shown in Fig. 2A. It can be clearly seen that all diffraction peaks in both samples corresponded to the standard diffraction of hexagonal ZnO (JCPDS-05-0664) and no other peaks exist, indicating that no by-products were synthesized during the reaction. Furthermore, in the case of both pure ZnO nanowire arrays and Ag–ZnO nanowire arrays, there is a very intense peak at 2q ¼ 34.52 for both products, which is assigned to the (002) plane of the ZnO nanowires, further illustrating the vertical orientation of ZnO nanowire arrays. However, note that although metallic Ag nanoparticles have been clearly observed in the SEM image (Fig. 1C and D), the diffraction peaks of Ag–ZnO nanowire arrays mainly correspond to ZnO nanowires, and no evident metallic Ag was detected probably due to a low content of metal silver in the as-prepared heterostructure, which is lower than the detection limit of routine XRD. In order to further investigate its composition and clarify the existence of metallic Ag nanoparticles in the sample, XPS analysis has been performed. The full XPS spectrum confirms the existence of Zn, O, Ag, and C elements (Fig. 3), demonstrating that Ag nanoparticles had been successfully deposited on the tips of the ZnO nanowires without any impurity. The inset in Fig. 3 presents the high-resolution spectra of Ag 3d in the hybrid nanocomposite, in which the Ag 3d 5/2 and Ag 3d 3/2 peaks appeared at a binding energy of 366.5 eV and 372.5 eV, respectively, confirming that the splitting of the 3d doublet was 6.0 eV, which reveals the metallic nature of silver.16 More specically, as compared with the standard values (about 368.2 and 374.2 eV for bulk Ag,15 respectively), Ag 3d 5/2 and Ag 3d 3/2 peaks shified identically to the lower binding energies, which should be ascribed to the transfer of electrons from Ag nanoparticles to ZnO nanowires at the interfaces.14 All these results are highly consistent with the observations made by other groups.17–21 Additionally, their elemental compositions were also evaluated by the EDS analysis (see Fig. S3, ESI†), confirming the existence of Zn and Ag (the peak of O was not observed due to the limitation of our instrument). The EDS result was in good agreement with the results of the above XPS analysis.