ZnO nanorods/Pt and ZnO nanorods/Ag heteronanostructure arrays with enhanced photocatalytic degradation of dyes

Abstract

Semiconductor–metal heteronanostructures provide an effective way to tailor the properties of semiconductor photocatalysts through promoting interfacial charge-transfer processes and enhancing charge separation. Here, a zinc substrate strategy has been developed for the solution-phase synthesis of well-defined ZnO nanorods/Pt and ZnO nanorods/Ag heteronanostructure arrays in high yields. The fabricated heteronanostructure arrays show significant structure-induced enhancements of photodegradation for rhodamine B and high photocatalytic stabilities, which are very attractive for real photocatalytic applications.

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1. Introduction

Due to the increasing environmental contamination caused by the adverse effects of industrialization, the development of renewable technologies for environmental remediation is highly desirable. The photocatalytic oxidation of mainly organic pollutants in water has been proved to be an effective technique for environmental remediation. To develop an efficient photocatalytic system, high-quality semiconductor-based materials have been actively studied over the past decades. Among a wide spectrum of semiconductors, ZnO has been extensively investigated as a photocatalyst due to some of its advantages such as an abundant source, non-toxicity and low cost.^1,2 However, a general limitation of single-component ZnO photocatalysts is the high recombination rate of the photogenerated charge carriers and low photostability due to photocorrosion.^3,4 On various frontiers of activity-enhanced photocatalysis, surface/interface-tuning strategies for modification of preliminary ZnO nanostructures have progressed at a fast pace, of which the design of ZnO–metal heteronanostructures is one of the promising approaches, where contact of the metal with the semiconductor can indirectly influence the energetics and interfacial charge-transfer processes in a favorable way.⁵ Enhancement of photocatalytic efficiency can be achieved by deposition of noble metal nanoparticles, such as Pt,^6–8 Au^6,9–14 or Ag^14–18 on the surface of ZnO nanostructures.

Among various noble-metal co-catalysts, Pt has attracted considerable attention as an efficient candidate for modifying a semiconductor due to its large work function. When it couples with ZnO, a Schottky barrier is formed and provides a direct rapid photogenerated electron transfer channel from an excited ZnO semiconductor to the Pt nanoparticles.^7,8 Another noble metal, Ag, is considered to be most favorable for industrial applications due to the advantages of high efficiency, low cost and easy preparation. In addition to the formation of a Schottky barrier at the semiconductor–Ag interface, which can effectively trap the photo-induced charges and inhibit their recombination, Ag can also enhance the photocatalytic activity by forming local surface plasmon resonance.^19–22 Substantial efforts have been devoted to obtain efficient ZnO/noble metal heteronanostructures, including ZnO/Pt and ZnO/Ag for improving photocatalytic performances. A typical method is to design a surface and/or interface deposition strategy for modifying the synthesized ZnO nanostructure; however, the self-nucleation of the second metal nanoparticles is often difficult to completely avoid, which will affect the efficiency and stability of the photocatalyst. Therefore, it is still a challenge to explore a rational and simple approach to fabricate a high-yield ZnO/noble-metal heteronanostructure with a special microstructure for high-activity photocatalytic applications.

Generally, powder-form nanosized photocatalysts often suffer from aggregation, deactivation and poor recyclability in an aqueous photocatalytic system. But photocatalysts with an array structure on the substrate not only can exhibit structurally related photocatalytic activities to a maximum extent due to the high distribution density and wide separation between the neighboring nanostructures, but also can be easily separated and recycled from the solution. In a previous study, we reported a preliminary study on Ag nanoparticles-modified ZnO nanorods on a zinc substrate and studied their powder-form photocatalytic properties detached from the zinc foil.²³ Herein, we further develop a general strategy to synthesize Pt or Ag-decorated ZnO nanorods arrays on zinc foil through simple chemical solution routes, aiming to suppress the self-nucleation of noble metal nanostructures and maximize the energy-harvesting and photocatalytic efficiency. The fabricated ZnO nanorods/Pt and ZnO nanorods/Ag heteronanostructure arrays show microstructure-induced enhancements of photodegradation for rhodamine B and high photocatalytic stabilities, which are very attractive for the real photocatalytic applications.

2. Experimental section

2.1 Synthesis of ZnO-NRs/Pt heteronanostructure array on zinc foil

Firstly, the synthesis of a ZnO nanorods (ZnO-NRs) array on zinc foil was performed according to our previous report.²³ In a typical synthesis, a zinc foil (99.9% 1 × 3 cm) washed with ethanol and distilled water respectively in an ultrasonic pool was inserted into the solution with 4 mL of 1,6-hexanediamine and 40 mL of distilled water in a Teflon-lined stainless steel autoclave of 50 mL capacity. The vessel was then heated at 180 °C for 5 h and cooled to room temperature naturally. The white precipitate covered on the zinc foil was washed several times with distilled water and ethanol and dried at 50 °C for 6 h.

Secondly, the ZnO nanorods/Pt nanoparticles (ZnO-NRs/Pt-NPs) heterostructure array was prepared as follows: 1 mL of H₂PtCl₆ solution (3.0 × 10⁻³ mol L⁻¹) was added into a mixed solvent of 10 mL 1,6-hexanediamine and 29 mL of distilled water to form a homogeneous solution under constant stirring for 5 min, then the solution was loaded into a 50 mL Teflon-lined stainless steel autoclave with the zinc foil covered by the ZnO-NRs inserted. The vessel was sealed and maintained at 180 °C for 5 h. The zinc foil covered with a gray-black precipitate was washed several times with distilled water and ethanol and dried for further characterization.

The hierarchical ZnO nanorods/ZnO nanosheets/Pt nanoparticles (ZnO-NRs/ZnO-NSs/Pt-NPs) heterostructure array was synthesized using 2 mL of H₂PtCl₆ solution and 28 mL of distilled water with other parameters kept constant.

2.2 Synthesis of ZnO-NRs/Ag heteronanostructure array on zinc foil

The synthesis of Ag nanoparticles on ZnO-NRs (ZnO-NRs/Ag-NPs) used a modification of Mirkin's method for Ag nanoplates.²⁴ In a typical synthesis, 0.95 mmol of PVP (K-30) and 0.16 mmol of AgNO₃ were dissolved in 100 mL of distilled water in a 150 mL glass reaction bottle with constant stirring. Then, 0.50 mmol of C₆H₅Na₃O₇·2H₂O and 0.29 mmol of NaBH₄ were added into the above mentioned solution. Subsequently, 1 mL of 30% H₂O₂ was slowly dropped into the solution. After another 1 h of agitation, the zinc foil covered by ZnO-NRs was inserted. Then the bottle was heated at 95 °C for 5 h and cooled to room temperature naturally. The resulting brownish-black product covered on the zinc foil was washed with distilled water and ethanol and dried for further characterization.

The synthesis of Ag nanoplates on ZnO-NRs (ZnO-NRs/Ag-NPLs) was conducted at 80 °C with other parameters kept constant.

2.3 Sample characterization

Field emission scanning electron microscope (FESEM) images were obtained using a Hitachi S-4800 operated at an accelerating voltage of 5.0 kV. For transmission electron microscopy (TEM) analysis a JEOL 2010 was used with an accelerating voltage of 200 kV. Photoluminescence (PL) spectra were detected with a LS-55 (PerkinElmer, USA) using a Xe laser as the excitation light source at 325 nm.

2.4 Photocatalytic experiments

Photocatalytic activities of the synthesized ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructure arrays were evaluated with rhodamine B (RhB) as a representative dye pollutant under UV irradiation. The heteronanostructure array was cut into 1 × 1.5 cm and placed in the bottom of a vial (25 × 40 mm) containing 5 mL of 10⁻⁵ M RhB aqueous solution and placed in the dark for 1 h to reach the adsorption/desorption equilibrium on the catalyst surface. Then, UV irradiation was carried out with a 100 W fluorescent Hg lamp (the strongest emission at 365 nm). At a given time interval of 2 minutes, the absorbance spectra of the solution were recorded using a UV-vis spectrophotometer (Shimadzu UV-2550), from which the concentration of RhB was measured by monitoring its characteristic absorption at 554 nm.

3. Results and discussion

Fig. 1a and b show the low- and high-magnification FESEM images of the ZnO-NRs/Pt-NPs heterostructure array, which clearly display that the secondary nanoparticles with average diameter of 20 nm are uniformly assembled on the entire side surface of the primary ZnO-NRs and aggregated into a large nanosphere on the top of many nanorods in comparison with the needle-like ZnO-NRs with smooth surfaces after the first hydrothermal treatment (Fig. S1†). The compositional information from the point-scan EDX in Fig. 1c shows Pt element is present with an atomic percentage of 6.53% along with Zn and O elements, providing powerful evidence that Pt element is successfully incorporated into the ZnO-NRs' surface. The HRTEM image in Fig. 1d along both sides of the interface clearly displays two different crystal parts, where one set of the planar spacing, about 0.263 nm, corresponds to the (002) plane of hexagonal wurtzite-type ZnO; another set of the fringes' spacing measuring ca. 0.227 nm corresponds to the (111) lattice spacing of cubic Pt, indicating that the Pt nanoparticles were indeed attached to the ZnO nanorod. The Pt nanoparticles grown on the ZnO-NRs are further confirmed by STEM measurement. The STEM image of a part of an individual nanorod in Fig. 2a shows there are many nanoparticles attached to the ZnO nanorod, further offering evidence for the formation of ZnO-NRs/Pt-NPs heteroarchitecture. The element mapping images in Fig. 2b–d clearly reveal the presence of Zn, O and Pt elements, respectively. Notably, the mapping of Pt element verifies that Pt nanoparticles are successfully attached to the ZnO nanorod.

Fig. 1 (a and b) The FESEM images of the ZnO-NRs/Pt-NPs heterostructure array with low- and high-magnification; (c) the EDX spectrum; (d) The HRTEM image of ZnO-NRs/Pt-NPs heterostructure, showing the characteristic lattice planes of ZnO and Pt.

Fig. 2 (a) The STEM image of a part of an individual ZnO-NRs/Pt-NPs heterostructure; (b–d) individual element mapping images of Zn, O and Pt, respectively.

When the amount of H₂PtCl₆ solution was doubled, the product was a hierarchical heteronanostructure. The low- and high-magnification FESEM images of the product in Fig. 3a and b exhibit the nanorods/nanosheets hierarchical nanostructure, and the high-magnification FESEM image in Fig. 3b also clearly reveals there were some nanoparticles attached to the nanorod. The compositional information from the point-scan EDX in Fig. 3c reveals Pt element with an atomic percentage of 10.86% was present along with Zn and O elements. The TEM image in Fig. 3d further shows that the product was a nanorod/nanosheet hierarchical structure. The HRTEM image of the nanosheet in Fig. 3e identified the dark/bright contrast, where different crystal parts are displayed, of which the planar spacing of about 0.281 nm corresponds to the (100) plane of hexagonal wurtzite-type ZnO; the fringes' spacing of 0.227 nm corresponds to the (111) lattice spacing of cubic Pt, indicating that the Pt nanoparticles with a size of 2–4 nm were uniformly attached to the ZnO nanosheet. Therefore, the hierarchical nanostructure was a ZnO-NRs/ZnO-NSs/Pt-NPs heteroarchitecture.

Fig. 3 (a and b) The FESEM images of the ZnO-NRs/ZnO-NSs/Pt-NPs heterostructure array with low- and high-magnification; (c) the EDX spectrum; (d) a typical TEM image of an individual ZnO-NRs/ZnO-NSs/Pt-NPs heterostructure, which clearly shows the nanorod and nanosheet hierarchical structure; (e) the HRTEM image of the nanosheet, showing the characteristic lattice planes of ZnO and Pt, respectively.

The ZnO-NRs/Ag-NPs heterostructure was also characterized by FESEM and TEM. The FESEM images in Fig. 4a and b show the Ag nanoparticles with average diameter of 10–15 nm uniformly covered on the primary ZnO-NRs. The point-scan EDX spectrum in Fig. 4c demonstrates that the atomic percentage of Ag is 8.19%, indicating the Ag element was successfully coupled on the surface of ZnO-NRs. The HRTEM image in Fig. 4d reveals two different crystal parts, of which the planar spacing of 0.263 nm corresponds to the (002) plane of hexagonal wurtzite-type ZnO; the fringes' spacing of 0.236 nm corresponds to the (111) lattice spacing of cubic Ag, further verifying that the Ag nanoparticles were attached to the ZnO nanorod. The typical STEM image of a part of an individual nanorod in Fig. 5a shows that many nanoparticles are homogeneously dotted on the ZnO nanorod. Fig. 5b–d show the elemental mapping investigation of the heterostructure, individually revealing three elements, Zn, O and Ag, clearly verifying that a ZnO-NRs/Ag-NPs heteroarchitecture was successfully obtained.

Fig. 4 (a and b) The FESEM images of the ZnO-NRs/Ag-NPs heterostructure array with low and high magnification; (c) the EDX spectrum; (d) the HRTEM image of ZnO-NRs/Ag-NPs heterostructure, showing the characteristic lattice planes of ZnO and Ag, respectively.

Fig. 5 (a) The STEM image of a part of an individual ZnO-NRs/Ag-NPs heterostructure; (b–d) individual element mapping images of Zn, O and Ag, respectively.

When the temperature was decreased to 80 °C with other parameters kept constant, the product was Ag nanoplates attached on ZnO-NRs as shown in Fig. 6a and b, of which the diameters of the nanoplates were mostly 30–40 nm. The point-scan EDX spectrum in Fig. 6c reveals that the atomic percentage of Ag is 4.79%, indicating less formation of Ag nanostructure at lower temperature. The TEM image in Fig. 6d further illustrates the Ag nanoplates were covered on the ZnO nanorod and the HRTEM image in Fig. 6e also shows two different crystal parts, which can be indexed to the (002) lattice spacing of hexagonal wurtzite-type ZnO and the (111) lattice spacing of cubic Ag.

Fig. 6 (a and b) The FESEM images of the ZnO-NRs/Ag-NPLs heterostructure array with low- and high-magnification; (c) the EDX spectrum; (d) A typical TEM image of an individual ZnO-NRs/Ag-NPLs heterostructure; (e) the HRTEM image of ZnO-NRs/Ag-NPLs heterostructure, showing the characteristic lattice planes of ZnO and Ag, respectively.

To elucidate the effect of the zinc substrate on the formation of the ZnO-NRs/noble-metal heteronanostructure, a comparison experiment for synthesis of a ZnO-NRs/Ag-NPs heterostructure using ZnO-NRs detached by ultrasonication from the zinc substrate was performed in the second solution-phase process with other parameters constant. After the reaction, ZnO nanorods attached to Ag nanoparticles were obtained but accompanied by some independent Ag nanoparticles (Fig. S2†). The result implied that the zinc substrate can effectively suppress the self-nucleation of noble metal nanostructures to obtain well-defined ZnO-NRs/noble-metal heteronanostructure arrays. In our case, the aligned needle-like ZnO-NRs standing on a zinc substrate with a large exposed surface possess a high distribution density and a wide separation between the neighboring nanorods, supplying a good opportunity for the uniform deposition of noble metal nanostructures onto the side surfaces and the top ends of ZnO-NRs via a hydrothermal approach, effectively suppressing the self-nucleation of noble metal nanostructures.

Here, the ZnO-NRs/Pt heteronanostructure arrays were prepared via a simple and novel approach, in which H₂PtCl₆ solution was added into a mixed solvent of 1,6-hexanediamine and distilled water with the zinc foil covered by ZnO-NRs inserted. It is considered that 1,6-hexanediamine molecules acted as the solvent and structure-directing coordination molecular template, which are the common roles of various kinds of short-chain water-soluble alkylamines.^25–29 Thus, it was inferred that the PtCl₆²⁻ ions were reduced to zero-valent Pt by Zn foil uncovered with ZnO-NRs. In this reaction system, the surface of the ZnO-NRs was activated by H⁺ ions ionized from chloroplatinic acid and rendered them amenable to PtCl₆²⁻ ions adsorption. Then, PtCl₆²⁻ ions were reduced to zero-valent Pt by Zn foil uncovered by ZnO-NRs and formed Pt nuclei around the ZnO-NRs, which were gradually grown via the coordination of 1,6-hexanediamine molecules. When the amount of H₂PtCl₆ doubled, the surface of the ZnO-NRs was etched by more H⁺ ions and regrew into ZnO nanosheets on ZnO-NRs through 1,6-hexanediamine preferentially bonding to the ZnO sheets' surface via surface ions. Moreover, the PtCl₆²⁻ ions adsorbed on the surface of ZnO-NRs and newly formed ZnO nanosheets were in situ reduced to Pt nanoparticles by Zn foil. To elucidate the effect of the zinc foil on the formation of ZnO-NRs/Pt-NPs heterostructure, an experiment without addition of 1,6-hexanediamine was performed in the second hydrothermal step keeping other parameters constant. After reaction, some ZnO-NRs attached to larger Pt-NPs were acquired (Fig. S3†), indicating that PtCl₆²⁻ ions can be reduced to Pt-NPs by Zn foil. A similar reduction mechanism has been clarified in ZnO–Pt hollow nanoparticles.^6,7 To clarify the effect of H₂PtCl₆ on realizing a hierarchical nanostructure, an experiment without H₂PtCl₆ was performed in the second solution-phase process with other parameters kept constant. After reaction, only nanorods with smooth surfaces were obtained (Fig. S4†). The result implied that a certain concentration of H₂PtCl₆ plays a particular role in the formation of a hierarchical nanostructure. Because no other additive agent, such as a surfactant or capping agent, was added, this novel and simple reaction to produce Pt nanoparticles on the surface of ZnO nanostructure makes the heterostructure have a clear surface, which is very attractive for high performance applications. As for the deposition of Ag nanostructures, the modified Mirkin's method for the preparation of Ag nanoplates has been successfully introduced to obtain a ZnO-NRs/Ag heteronanoarchitecture. Herein, the surface of the synthesized ZnO-NRs was functionalized with citrate ions,¹² and the capping agent of PVP molecules can also be adsorbed on the surface of the ZnO-NRs and decrease interfacial energies, which favors the incorporation of Ag nanostructures over the surface of the ZnO-NRs. Only Ag nanoparticles covered on ZnO-NRs were acquired at 95 °C, whereas Ag nanoplates dotted on ZnO-NRs were obtained at 80 °C, indicating the lower temperature favored the anisotropic growth of Ag nanoplates. Undoubtedly, our work indicates that the Zn substrate strategy is indeed a generalized strategy for preparing high-yield ZnO/noble-metal heteronanostructure arrays.

The optical properties of the ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructures were studied by the combined analyses of the UV-visible diffuse reflectance spectra and the photoluminescence (PL) spectra. As shown in Fig. 7a, the spectrum of ZnO-NRs shows a strong UV absorption band characteristic of the wide band gap materials of ZnO semiconductors along with a small absorption tail in the visible region. The ZnO-NRs/Ag-NPs and ZnO-NRs/Ag-NPLs samples individually exhibit an additional absorption peak at 452 nm and 445 nm in contrast to pure ZnO-NRs, which is normally attributed to the typical surface plasmon absorption of silver nanostructures.^30–32 The absorption results were also consistent with the visible sample color shift from white to brownish-black. In addition, the ZnO-NRs/Ag-NPs heterostructure absorbed more visible light than ZnO-NRs/Ag-NPLs, which may be due to the higher Ag content. The UV-visible spectra of ZnO/Pt heteronanostructures both show increased absorption at visible wavelengths than pure ZnO-NRs,³³ consistent with the visible sample color shift from white to gray-black. The extended absorbance of the ZnO-NRs/Ag and ZnO-NRs/Pt heteronanostructures, particularly ZnO-NRs/Ag heteronanostructures in the visible range, is of practical importance, which makes them suitable for efficient utilization of sunlight or visible light to generate more photoexcited charges in the photocatalytic degradation reactions. The PL emission spectra were investigated to reveal the properties of charge carrier trapping, immigration and transfer in the ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructures. Fig. 7b displays the room temperature PL emission spectra of the four heteronanostructures as well as pure ZnO-NRs. All of the PL spectra possess two similar independent peaks, where the peak in the shorter wavelength region corresponds to the near-band-edge emission, whereas the peak at a longer wavelength can be assigned to the defect-related emission. An enhancement in the band emission and reduction in the defect emission of ZnO-NRs was observed after incorporation of a Pt or Ag nanostructure. In the ZnO-NRs/Pt heteronanostructures, the defect level of ZnO lies very close to the Fermi level of Pt; the electrons can easily transfer from the defect level to the Fermi level of Pt, and an enhanced interband transition occurred, where electrons from the CB of ZnO combined with holes in the VB, resulting in an enhancement in the band emission and a subsequent reduction in the defect emission.³⁴ As for the ZnO-NRs/Ag heteronanostructures, Ag atoms can occupy Zn atom sites in the lattice of ZnO; when the incident light excites the carriers in the sample, the photocarriers may escape more easily from Ag ions than from Zn ions, which results in a faster diffusion of excitons in ZnO and more electron–hole pairs, leading to increased exciton recombination, and thus the intensity of the UV emission is increased. On the other hand, Ag coupling causes the migration of Ag atoms to the Zn vacancies, which in turn decreases the intensity of the defect emission.^35,36 Slight blue shifts of the near-band-edge emissions for the heteronanostructures are observed, which may be related to the excited electrons accumulated in the metal particles possessing higher energy, leading to the electron/hole recombination-emitted shorter-wavelength luminescence. The PL spectra demonstrate that the heteronanostructures can suppress the recombination rate of electron–hole pairs, thus facilitating the charge separation.³⁷

Fig. 7 The optical properties of the ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructures as well as pure ZnO-NRs. (a) The UV-visible diffuse reflectance spectra; (b) the PL spectra.

The photocatalytic activities of the ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructure arrays were evaluated under UV light irradiation using RhB as a model dye pollutant in aqueous solutions, and the relevant data for the ZnO-NRs array and the absence of catalyst are also presented as a contrast. The characteristic absorbance of RhB in aqueous solution at 554 nm decreases with irradiation time, and the decoloration rate of RhB in the presence of the ZnO-NRs/Ag-NPs heterostructure array reached 96.8% after 12 min, 94.5% with ZnO-NRs/Ag-NPLs, 90.6% with ZnO-NRs/ZnO-NSs/Pt-NPs, and 87.5% with ZnO-NRs/Pt-NPs, but 68.7% of RhB molecules were decomposed with ZnO-NRs in the same period (Fig. 8). There was only slight degradation with the blank test, indicating that photoinduced self-sensitized photolysis can be ignored in comparison with the photocatalysis caused by catalysts. Clearly, the ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructure arrays all have superior photocatalytic activities to that of the ZnO-NRs array, and the photocatalytic performances of ZnO-NRs/Ag arrays are better than those of ZnO-NRs/Pt arrays. Moreover, the ZnO-NRs/Ag-NPs array exhibits slightly higher photocatalytic activity compared to ZnO-NRs/Ag-NPLs, and the ZnO-NRs/ZnO-NSs/Pt-NPs array has slightly better photocatalytic performance than that of ZnO-NRs/Pt-NPs. The results indicated that the photocatalytic activity of a ZnO-NRs array could be improved by coupling with a Pt or Ag nanostructure, and the microstructure and content of the secondary metal nanostructure can also affect the photocatalytic performance. The enhanced photocatalytic activities of the ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructure arrays benefited from the presence of a ZnO-metal nano-nano heterojunction, which formed a Schottky barrier via Fermi level equilibration between the metal and the semiconductor ZnO, promoting the separation of photogenerated electrons and holes. Moreover, the metal Ag nanostructures can produce surface plasmon resonance with resonant photon-induced collective oscillation of valence electrons, which could enhance the concentration of photogenerated charge carriers, benefiting the photocatalytic activity.^19–22 The different photocatalytic performances of ZnO-NRs/Ag-NPs and ZnO-NRs/Ag-NPLs, as well as ZnO-NRs/ZnO-NSs/Pt-NPs and ZnO-NRs/Pt-NPs, were mainly related to the amount of heterojunctions and the product's surface area (the latter particularly for the ZnO-NRs/ZnO-NSs/Pt-NPs and ZnO-NRs/Pt-NPs samples), which can affect the charge separation and light absorption, respectively.

Fig. 8 (a) UV-visible absorption spectra of degradation RhB solution by ZnO-NRs/Pt, ZnO-NRs/Ag heteronanostructure arrays and pure ZnO-NRs array as well as the blank, where C₀ and C are the initial concentration after the adsorption equilibrium and the reaction concentration of RhB, respectively.

To further investigate the photocatalytic performance of the as-prepared arrays in the visible light region, photocatalytic degradation experiments were carried out using a 300 W Xe lamp irradiation. The results are shown in Fig. S5.† The photocatalytic degradation rate of RhB reached 86.7% with the ZnO-NRs/Ag-NPs array after 3 h, 83.6% with ZnO-NRs/Ag-NPLs, 72.0% with ZnO-NRs/ZnO-NSs/Pt-NPs, and 61.8% with ZnO-NRs/Pt-NPs, but 37.0% of RhB molecules were decomposed with ZnO-NRs in the same period. As a comparison, the relevant data in the absence of catalyst was also tested, which revealed the decomposition of RhB was about 18.3%, indicating that photoinduced self-sensitized photolysis can be induced by photo-absorption of RhB itself after a long time of irradiation. But the degradation rate was remarkably enhanced when a ZnO-NRs/Pt or ZnO-NRs/Ag heteronanostructure array photocatalyst was added. The results suggested the as-prepared ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructure arrays, particularly ZnO-NRs/Ag arrays, can also be used as visible-light-driven photocatalysts.

To discover the dye photosensitization in the photodegradation of these colored organics, a comparative experiment of the photocatalytic degradation of colorless organics, with phenol, was carried out with a ZnO-NRs/Ag-NPs array under UV irradiation. There was about 41.6% photodegradation after irradiation within the test period of 12 min (Fig. S6†), providing evidence for the photodegradation of colorless organics. However, the clear lower photodegradation rate of phenol than that of RhB demonstrates the indirect colored dye photosensitization of dye degradation by electron injection from dye to catalyst also plays a crucial role in the degradation of RhB under UV irradiation.

Important considerations for the application of a photocatalyst are its stability and separation. However, as a photocatalyst, one of the main disadvantages of ZnO is its poor photostability due to photoinduced dissolution. But for ZnO/Pt and ZnO/Ag heteronanostructures, the vacant sites can be occupied by Pt and Ag, which effectively inhibit the photocorrosion behavior. To identify the stabilities of the highly photocatalytic behaviors of the ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructure arrays, recycling experiments on the photocatalytic degradation of RhB were carried out taking ZnO-NRs/Ag-NPs and ZnO-NRs/ZnO-NSs/Pt-NPs arrays as examples (Table S1†). The two heteronanostructure arrays both exhibited considerably higher recycling stabilities than that of the pure ZnO-NRs array, demonstrating the heteronanostructures are stable and invulnerable to photocorrosion during the photocatalytic degradation of organic species, which is particularly attractive for real applications. Evidently, the as-prepared heteronanostructure arrays as photocatalysts can effectively prevent the possible agglomeration and microstructure destruction that powder-form nanosized photocatalysts often suffer and therefore can take full advantage of the structurally related photocatalytic activities. Moreover, they have the advantages of easy separation and recycling from the solution.

The electron-transfer processes of ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructures in the photodegradation of RhB are analyzed and presented in Fig. 9. For a ZnO-NRs/Pt heteronanostructure, a Schottky barrier is formed between ZnO and the Pt nanostructure due to the larger work function of Pt related to ZnO, which provides a direct rapid photogenerated electron transfer channel from the excited ZnO to the Pt nanoparticles. As for a ZnO-NRs/Ag heteronanostructure, the Fermi level of ZnO is lower than that of Ag because of the larger work function of ZnO, which leads to electron transfer from Ag to ZnO until the two systems attain equilibrium and form a new energy level. The photoexcited electrons can also move from ZnO to Ag because the bottom energy level of the CB of ZnO is higher than the newly formed Fermi energy level of Ag–ZnO. Under UV light illumination, electrons can transfer from the dye in its singlet excited state to the CB of ZnO, metal nanoparticles and the shallow trap levels in the band gap of ZnO.³⁸ In addition, the electrons and holes generated by ZnO were separated, of which the electrons can be transferred from the CB of ZnO to the Fermi level of Pt or Ag. Therefore, the ZnO/Pt and ZnO/Ag heteronanostructures can form a Schottky barrier at the semiconductor–metal interface with Pt or Ag acting as electron sinks, effectively trapping the photo-induced electrons.^{6,8,15,39–43} The transferred electrons can be trapped by the adsorbed O₂ molecules to give ˙O²⁻ radicals,⁴⁴ and holes generated in ZnO can be scavenged by the ubiquitous H₂O molecules to yield ˙OH radicals, both of which are the main free radicals that decompose organic compounds.⁴⁵ Therefore, Pt or Ag has a role in utilizing the captured electrons and thereby the free holes in this system containing a sensitizing dye, thus preventing their recombination. Moreover, the metal Ag can produce surface plasmon resonance with resonated photon-induced collective oscillation of valence electrons, which could enhance the concentration of photogenerated charge carriers.

Fig. 9 Schematic diagram of the electron-transfer processes of ZnO-NRs/Pt (a) and ZnO-NRs/Ag (b) heteronanostructures in photodegradation of RhB. The data are partially from ref. 23, 34 and 40–42.

4. Conclusions

A simple and versatile method to fabricate high-yield ZnO-NRs/Pt and ZnO-NRs/Ag heteronanostructure arrays has been developed by chemical solution deposition of Pt or Ag nanostructures on a ZnO-NRs array on zinc foil. These heteronanostructure arrays show improved photocatalytic activities in the decomposition of RhB dye, which are associated with charge separation between ZnO-NRs and Pt or Ag nanostructure as well as the plasmon enhanced effect of the Ag nanostructure. Furthermore, excellent photocatalytic recycled stabilities were discovered in these heteronanostructures, making them have great potential as high-performance photocatalysts in practical applications.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (21201007).

Notes and references

1. J. Becker, K. R. Raghupathi, J. S. Pierre, D. Zhao and R. T. Koodali, J. Phys. Chem. C, 2011, 115, 13844–13850.
2. L. P. Xu, Y.-L. Hu, C. Pelligra, C.-H. Chen, L. Jin, H. Huang, S. Sithambaram, M. Aindow, R. Joesten and S. L. Suib, Chem. Mater., 2009, 21, 2875–2885.
3. M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
4. H. J. Zhang, G. H. Chen and D. W. Bahnemann, J. Mater. Chem., 2009, 19, 5089–5121.
5. S. T. Kochuveedu, Y. H. Jang and D. H. Kim, Chem. Soc. Rev., 2013, 42, 8467–8493.
6. H. B. Zeng, W. P. Cai, P. S. Liu, X. X. Xu, H. J. Zhou, C. Klingshirn and H. Kalt, ACS Nano, 2008, 2, 1661–1670.
7. H. B. Zeng, P. S. Liu, W. P. Cai, S. K. Yang and X. X. Xu, J. Phys. Chem. C, 2008, 112, 19620–19624.
8. J. Yuan, E. S. G. Choo, X. Tang, Y. Sheng, J. Ding and J. Xue, Nanotechnology, 2010, 21, 185606.
9. Q. Wang, B. Y. Geng and S. Z. Wang, Environ. Sci. Technol., 2009, 43, 8968–8973.
10. N. Udawatte, M. Lee, J. Kim and D. Lee, ACS Appl. Mater. Interfaces, 2011, 3, 4531–4538.
11. W. W. He, H.-K. Kim, W. G. Wamer, D. Melka, J. H. Callahan and J.-J. Yin, J. Am. Chem. Soc., 2014, 136, 750–757.
12. M. D. L. R. Peralta, U. Pal and R. S. Zeferino, ACS Appl. Mater. Interfaces, 2012, 4, 4807–4816.
13. S. Sarkar, A. Makhal, T. Bora, S. Baruah, J. Duttab and S. K. Pal, Phys. Chem. Chem. Phys., 2011, 13, 12488–12496.
14. P. Fageria, S. Gangopadhyayb and S. Pande, RSC Adv., 2014, 4, 24962–24972.
15. S. A. Ansari, M. M. Khan, M. O. Ansari, J. Lee and M. H. Cho, J. Phys. Chem. C, 2013, 117, 27023–27030.
16. Q. Deng, X. W. Duan, D. H. L. Ng, H. B. Tang, Y. Yang, M. G. Kong, Z. K. Wu, W. P. Cai and G. Z. Wang, ACS Appl. Mater. Interfaces, 2012, 4, 6030–6037.
17. C. L. Ren, B. F. Yang, M. Wu, J. Xu, Z. P. Fu, Y. Lv, T. Guo, Y. X. Zhao and C. Q. Zhu, J. Hazard. Mater., 2010, 182, 123–129.
18. S. Kuriakose, V. Choudhary, B. Satpatib and S. Mohapatra, Phys. Chem. Chem. Phys., 2014, 16, 17560–17568.
19. L. L. Sun, W. Wu, S. L. Yang, J. Zhou, M. Q. Hong, X. H. Xiao, F. Ren and C. Z. Jiang, ACS Appl. Mater. Interfaces, 2014, 6, 1113–1124.
20. J. Y. Xiong, Z. Li, J. Chen, S. Q. Zhang, L. Z. Wang and S. X. Dou, ACS Appl. Mater. Interfaces, 2014, 6, 15716–15725.
21. Y. Z. He, P. Basnet, S. E. H. Murph and Y. P. Zhao, ACS Appl. Mater. Interfaces, 2013, 5, 11818–11827.
22. Z. J. Zhang, W. Z. Wang, E. P. Gao, S. M. Sun and L. Zhang, J. Phys. Chem. C, 2012, 116, 25898–25903.
23. Z. C. Wu, C. R. Xu, Y. Q. Wu, H. Yu, Y. Tao, H. Wan and F. Gao, CrystEngComm, 2013, 15, 5994–6002.
24. G. S. Métraux and C. A. Mirkin, Adv. Mater., 2005, 17, 412–415.
25. W. Liu, N. Wang and R. M. Wang, Nano Lett., 2011, 11, 2983–2988.
26. J. Cravillon, R. Nayuk, S. Springer, A. Feldhoff, K. Huber and M. Wiebcke, Chem. Mater., 2011, 23, 2130–2141.
27. W. T. Yao, S. H. Yu, S. J. Liu, J. P. Chen, X. M. Liu and F. Q. Li, J. Phys. Chem. B, 2006, 110, 11704–11710.
28. Z. C. Wu, Y. Q. Wu, T. H. Pei, H. Wang and B. Y. Geng, Nanoscale, 2014, 6, 2738–2745.
29. Z. C. Wu, H. Yu, L. Kuai, H. Wang, T. H. Pei and B. Y. Geng, J. Colloid Interface Sci., 2014, 426, 83–89.
30. C. Pacholski, A. Kornowski and H. Weller, Angew. Chem., Int. Ed., 2004, 43, 4774–4777.
31. C. Q. Chen, Y. H. Zheng, Y. Y. Zhan, X. Y. Lin, Q. Zheng and K. M. Wei, Dalton Trans., 2011, 9566–9570.
32. Y. C. Lu, Y. H. Lin, D. J. Wang, L. L. Wang, T. F. Xie and T. F. Jiang, J. Phys. D: Appl. Phys., 2011, 44, 315502.
33. P. Pawinrat, O. Mekasuwandumrong and J. Panpranot, Catal. Commun., 2009, 10, 1380–1385.
34. J. M. Lin, H. Y. Lin, C. L. Cheng and Y. F. Chen, Nanotechnology, 2006, 17, 4391–4394.
35. Y. Zhang, Z. Y. Zhang, B. X. Lin, Z. X. Fu and J. Xu, J. Phys. Chem. B, 2005, 109, 19200–19203.
36. J. Xu, Z. Y. Zhang, Y. Zhang, B. X. Lin and Z. X. Fu, Chin. Phys. Lett., 2005, 22, 2031–2034.
37. Y. Liu, L. Yu, Y. Hu, C. F. Guo, F. M. Zhang and X. W. Lou, Nanoscale, 2012, 4, 183–187.
38. R. Georgekutty, M. K. Seery and S. C. Pillai, J. Phys. Chem. C, 2008, 112, 13563–13570.
39. W. W. Lu, S. Y. Gao and J. J. Wang, J. Phys. Chem. C, 2008, 112, 16792–16800.
40. W. W. Lu, G. S. Liu, S. Y. Gao, S. T. Xing and J. J. Wang, Nanotechnology, 2008, 19, 445711.
41. Y. H. Zheng, L. R. Zheng, Y. Y. Zhan, X. Y. Lin, Q. Zheng and K. M. Wei, Inorg. Chem., 2007, 46, 6980–6986.
42. Y. H. Zheng, C. Q. Chen, Y. Y. Zhan, X. Y. Lin, Q. Zheng, K. M. Wei and J. F. Zhu, J. Phys. Chem. C, 2008, 112, 10773–10777.
43. Z. H. Zhang, M. F. Hossain, T. Arakawa and T. Takahashi, J. Hazard. Mater., 2010, 176, 973–978.
44. J. Ryu and W. Choi, Environ. Sci. Technol., 2004, 38, 2928–2933.
45. Y. F. Sun, B. Y. Qu, Q. Liu, S. Gao, Z. X. Yan, W. S. Yan, B. C. Pan, S. Q. Wei and Y. Xie, Nanoscale, 2012, 4, 3761–3767.

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Footnote

† Electronic supplementary information (ESI) available: FESEM images of the needle-like ZnO-NRs array, the ZnO-NRs/Pt-NPs product synthesized without addition of 1,6-hexanediamine and the ZnO-NRs product synthesized without addition of H₂PtCl₆ solution in the second reaction step, the results of photocatalytic degradation of RhB under visible-light irradiation, cycling results of photocatalytic degradation of RhB with ZnO-NRs/Ag-NPs, ZnO-NRs/ZnO-NSs/Pt-NPs heterostructure arrays and ZnO-NRs array are available in the ESI. See DOI: 10.1039/c4ra10753e

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