Controlled preparation of Ag–Cu2O nanocorncobs and their enhanced photocatalytic activity under visible light

Accepted Manuscript Title: Controlled preparation of Ag-Cu2 O nanocorncobs and their enhanced photocatalytic activity under visible light Author: Siyuan Yang Shengsen Zhang Hongjuan Wang Hao Yu Yueping Fang Feng Peng PII: DOI: Reference:

S0025-5408(15)00319-0 http://dx.doi.org/doi:10.1016/j.materresbull.2015.04.061 MRB 8206

To appear in:

MRB

Received date: Revised date: Accepted date:

24-2-2015 28-4-2015 30-4-2015

Please cite this article as: Siyuan Yang, Shengsen Zhang, Hongjuan Wang, Hao Yu, Yueping Fang, Feng Peng, Controlled preparation of Ag-Cu2O nanocorncobs and their enhanced photocatalytic activity under visible light, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.04.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Controlled preparation of Ag-Cu2O nanocorncobs and their enhanced photocatalytic activity under visible light

Siyuan Yang,a Shengsen Zhang,a,b Hongjuan Wang,a Hao Yu,a Yueping Fangb and Feng Peng*,a

a

School of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou, Guangdong, 510640, China. b

College of Science, South China Agricultural University, Guangzhou, 510642, China

Corresponding address. Feng Peng [Post address] School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China Fax: (+86) 20 87114916; Tel: (+86) 20 87114916; E-mail: [email protected]

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Graphical abstract

The corncob-like Ag-Cu2O nanostructure with suitably exposed Ag surface exhibited much higher photocatalytic activity than Ag@Cu2O nanocables and Cu2O nanowires.

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Highlights ►Ag-Cu2O nanocorncobs have been controllably prepared by a simple synthesis. ►The possible formation mechanism of Ag-Cu2O has been studied. ►Ag-Cu2O exhibits noticeable improved photocurrent compared with the pure Cu2O NWs. ►Ag-Cu2O with suitably exposed Ag surface shows much higher photocatalytic activity.

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Abstract

Novel corncob-like nano-heterostructured Ag-Cu2O photocatalyst has been controllably prepared by adjusting the synthetic parameters, and the possible formation mechanism has been also studied. The photoelectrochemical and photocatalytic performances demonstrated that the asprepared Ag-Cu2O nanocorncobs exhibited higher photocatalytic activity than both pure Cu2O nanowires and cable-like Ag@Cu2O nano-composites. It was concluded that Ag-Cu2O nanocorncobs with suitably exposed Ag surface not only effectively inhibit the recombination of electron-hole pairs but also suitably increase the active sites of electronic conduction, and thus increasing the photocatalytic activity under visible light irradiation.

Keywords

A. Nanostructure; A. Composite; B. Chemical synthesis; C. Transmission electron microscopy; D. Catalytic properties

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1. Introduction With a suitable conduction band (CB) of ~ 0.7 eV and small band gap of ~ 2.0 eV which directly result in an efficient harvesting and utilization of solar energy, p-type Cu2O is thought to be one of the most promising candidate photocatalysts for hydrogen generation and water treatment under visible light irradiation [1-4]. Meanwhile, it is also an earth-abundant, inexpensive, environment-friendly and practically feasible solar energy conversion material [5-7]. However, there are indeed some drawbacks, such as the poor stability and the fast electron-hole recombination [8-10], which severely hinder the theoretically excellent performance of Cu2O for the practical application. To satisfy the demands for photocatalyst with high photoactivity and good reusability, many researchers have extensively focused on how to enhance the utilizable efficiency of photogenerated electrons and holes. Typically, forming a n-p junction, such as Cu2O-ZnO, Cu2O-TiO2 and Cu2O-WO3, could effectively promote the transport of photogenerated electrons from p-type semiconductor to n-type semiconductor, and largely enhanced the photocatalytic activity [11-13]. Recently, the different shape Cu2O materials, such as polyhedron Cu2O, branched Cu2O nanowires and multi-shelled Cu2O hollow spheres [14-16], have attracted considerable attentions due to their high efficiency along with a good stability. Early in 1998, Domen et al first reported that Cu2O is capable of photocatalytic decomposition of water into H2 and O2 under visible light irradiation [17]. But one year later, the stability of Cu2O under illumination as a crucial issue was put forward by de Jongh et al [18]. A significant reason for the photocorrosion of Cu2O is the reductive decomposition according to the reaction (1) [19]. Cu2O + 2e- + 2H+ → Cu + H2O 5

(1)

Based on this reaction, it would be necessary to accelerate the transport of photogenerated electrons on Cu2O, which will not only help suppress the photo-corrosion and inhibit the recombination of electron-hole pairs but also effectively increase the photocatalytic activity. Unfortunately, it is still a major challenge to develop a novel Cu2O composite material which would favour electron transfer and enhance the photocatalytic activity of Cu2O. Recently, silver nanowires have been reported as a good electron acceptor which could effectively facilitate the electron transport in the core-shell Ag@Ag3PO4 nano-photocatalyst [20]. It's probably no coincidence that, Sciacca and Xiong et al have recently synthesized the similar heterostructure of Ag@Cu2O core-shell nanocomposite and confirmed the silver nanowires also played a role in promoting the rapid transfer of photo-generated electrons on the conduction band of Cu2O [2122]. However, Bi et al further proved that the necklace-like Ag nanowires/Ag3PO4 cube heterostructured photocatalyst exhibited much higher activity than the cable-like Ag@Ag3PO4 core-shell coaxial nanowires [23]. These results indicate that the combining form between silver and semiconductor may largely influence the photocatalytic performance of the composite materials. Therefore, it gives us much inspiration to find a more efficient combining structure of Ag and Cu2O which may exhibit further enhanced photocatalytic activity. Based on the above point of view, we first designed and fabricated a novel heterostructured photocatalyst of corncob-like Ag nanowires (NWs) and Cu2O nanoislands composites, which was denoted as Ag-Cu2O nanocorncobs. To comparably investigate the advanced achievements of Ag-Cu2O nanocorncobs, the cable-like Ag NWs and Cu2O core-shell composite was also synthesized by adjusting the synthetic parameters, which was denoted as Ag@Cu2O nanocables. The structures and morphologies of the obtained samples were characterized by high-resolution transmission electron microscopic and field-emission scanning electron microscopic techniques.

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Their optical-electrical properties and photocatalytic activity were investigated. It was found that Ag-Cu2O nanocorncobs with suitably exposed Ag NWs exhibited much higher photocatalytic properties than Ag@Cu2O nanocables and pure Cu2O nanowires. Finally, a possible mechanism for the inhibition of photo-corrosion and the enhancement of photocatalytic activity was also investigated. 2. Experimental section 2.1 Preparation of materials All the reagents were used as received without any further purification. (1) Synthesis of Ag nanowires (Ag NWs) Ag nanowires were synthesized as previously reported [24]. Typically, 0.2 g of polyvinylpyrrolidone (PVP, Mw = 360 000) was first added to 25 mL of ethylene glycol (EG) in a 100 ml round flask and completely dissolved using magnetic stirring at room temperature. Afterwards, 0.25 g of silver nitrate (AgNO3) was added to the PVP solution. Finally, 3.25 ml of a FeCl3 solution (600 mM in EG) was dumped into the mixture and stirred for one or two minutes. The mixture was then immediately transferred into a reactor preheated at 130 oC to grow Ag NWs for 5 h. (2) Synthesis of Ag@Cu2O nanocables After the formation of Ag NWs, 10 ml of cupric acetate (Cu(Ac)2, 0.1 M) and 5 ml of glucose (0.36 M) were directly added to the as-prepared Ag NWs solution. After 5 min of vigorous stirring, the mixture solution was then transferred into an oil bath at 100 oC. The reduction reaction of Cu2+ was continued for 1 h to make sure that all the Cu2+ turned into Cu2O. (3) Synthesis of Ag-Cu2O nanocorncobs 7

The synthesis of Ag-Cu2O nanocorncobs was basically as the same as that of Ag@Cu2O nanocables except for some changes. Firstly, the synthesized Ag NWs were centrifuged and washed with ethanol and water for four or five times, respectively, and then re-dispersed in a 25 ml of deionized H2O. Sequentially, 40 ml of PVP solution (0.02 g/ml) were injected into the Ag NWs-water solution. After 5 min stirring, 10 ml of Cu(Ac)2 (0.1 M) and 5 ml of glucose (0.36 M) were added to the mixture solutions of Ag NWs and PVP, and then 2.5 ml of NaOH (1 M) were dropped to the mixture solutions under continually stirring. Finally, the formed mixtures were reacted in an oil bath at 80 oC for 1 hour. (4) Synthesis of Cu2O nanowires (Cu2O NWs) Cu2O NWs with uniform diameter (150-180 nm) were prepared through the reduction of cupric acetate using pyrrole as the reductant in aqueous solutions under hydrothermal conditions. Typically, 0.3 g of Cu(Ac)2 was dissolved in 60 mL of deionized water. Afterward, 10 mL of an aqueous solution of pyrrole (0.10 M) was added to this solution. The precursor mixture was transferred to a 100 mL autoclave and maintained at 180 oC for 12 h, and subsequently cooled to ambient temperature naturally. All the obtained samples were washed with distilled water and ethanol for several times to remove excess PVP and EG via centrifugation, and dried at 60 oC in a vacuum box. 2.2 Catalyst characterization The morphologies of the as-prepared samples were obtained in a field-emission scanning electron microscope (SEM, LEO 1530VP) and transmission electron microscope (TEM, FEI Tecnai 20). The element contents of the as-prepared samples were analyzed by energy dispersive X-ray spectroscopy (EDX, JEM-2100). X-ray diffraction (XRD) patterns of samples were

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recorded on an X-ray diffractometer (D/max-IIIA, Japan) using Cu Ka as the radiation source. The UV-vis absorption spectra of all samples were conducted by a U3010 spectrophotometer (Hitachi, Japan) with an integrated sphere attachment. For UV-vis absorption test, the amount of all samples was the same (0.1 g). 2.3 Photocurrent test To prepare the working electrodes, 5 mg of the sample was first dispersed into a mixture of 2.0 ml ethanol and 0.01 ml Nafion, and then sonicated for 30 min to form a slurry, and finally the resulting slurry was dropped on FTO glass (2 × 3 cm, 10 Ω/square, Nippon Sheet Glass, Japan). The obtained samples were dried at 80 °C for 24 h in vacuum. Photocurrent measurements were carried out in a standard electrochemical workstation (CHI660D, Chenhua, Shanghai) coupled with a Xe lamp (PLS-SXE300UV, TrustTech, China). The wavelengths of the incident light were greater than 400 nm through a UV-400 filter. The intensity of the incident light was 150 mW/cm2. A 0.1 M NaNO3 solution was used as supporting electrolyte. 2.4 Photocatalytic test. The photocatalytic reaction was conducted in a 200 mL cylindrical glass vessel fixed in the XPA-II photochemical reactor (Nanjing Xujiang Machine-electronic Plant). A 500 W Xe lamp was used as the simulated solar light source (UV-vis light), and a house-made filter was mounted on the lamp to eliminate infrared irradiation. The visible light was obtained using the cut-off filter. The cut-off filter was made up of 1 M sodium nitrite solution, which can absorb the light with wavelength under 400 nm. Methyl orange (MO) with a concentration of 20 mg/L was used as contamination. In order to obtain an optimally dispersed system and reach complete adsorption/desorption equilibration, 20 mg photocatalyst powder was dispersed in 200 mL reaction solutions by sonicating for 30 min, and then the suspension was magnetically stirred in 9

dark for 1 h. During the photocatalytic reaction, air was blown into the reaction medium at a flow rate of 100 mL/min. At regular intervals, 5 mL of the suspension was filtered and then centrifuged. The concentration of the remaining MO was measured by its absorbance (A) at 464 nm with a Hitachi UV-3010 spectrophotometer. The decolourization ratio of MO could be calculated by (C0-C)/C0×100%. 3. Results and discussion 3.1. Sample characterizations Fig. 1A shows the XRD patterns of the two kinds of Ag and Cu2O composites. It is found that both Ag NWs and Cu2O in the two samples of Ag-Cu2O nanocorncobs and Ag@Cu2O nanocables are well crystallized, and these diffraction peaks can be easily indexed to facecentered cubic (fcc) Ag (JCPDS 65-2871) and cubic Cu2O (JCPDS 05-0667). It is also worth noting that the XRD peaks of Cu2O from Ag@Cu2O are broader than that from Ag-Cu2O, which is due to the smaller size of the Cu2O nanocrystallites on the surface of Ag NWs in Ag@Cu2O nanocables. The EDX pattern from Ag-Cu2O nanocorncobs (the result of Ag@Cu2O nanocables is not given here) reveals that the as-prepared samples are composed of Ag, Cu and O elements (carbon comes for the based material of the conductive fabric), and the standardless atom counting rate of Cu vs O in the selected region is nearly 2:1 which matches well with the molecular structure of Cu2O (Fig. 1C). The normalized mass contents of Cu2O in Ag-Cu2O nanocorncobs and Ag@Cu2O nanocables are 56.7% and 57.5%, respectively. No peaks for other impurities are found in both XRD and EDX patterns, revealing the high purity of all samples. The morphology of the as-prepared samples was characterized by SEM and TEM. As shown in Fig. 2A, the synthesized Ag NWs have a very smooth surface with an average diameter of ~ 200 nm. Large-scale SEM images for Ag-Cu2O and Ag@Cu2O were given in Figs. 2B and 2C, 10

which directly show the uniformity of the as-prepared samples. To clearly observe the detailed structure of the obtained samples, a set of magnified SEM and TEM images were given in Figs. 2D-2G. From Figs. 2D and 2F, it can be seen that Cu2O nanoislands with an average diameter of 70 nm were loaded on the surface of Ag NWs to form corncob-like Ag-Cu2O nanostructure. Figs. 2E and 2G clearly show that the Cu2O nanoparticles fully covered the Ag NWs surface to form cable-like Ag@Cu2O nanostructure, and the thickness of Cu2O layer is about 50 nm. Compared the SEM and TEM images of the two as-synthesized composites, the main difference exists in the exposure

degree

of

Ag

NWs

surface,

which

would

apparently

influence

their

photoelectrochemical properties (as discussed later). The samples have also been further examined by the high resolution transmission electron microscopy (HRTEM). As shown in Figs. 3A and 3B, the interplanar d spacings of silver nanowires are 0.236 and 0.204 nm which correspond to those of the (111) and (200) lattice planes of face-centered cubic silver, and the spacing of adjacent fringes of Cu2O is 0.214 nm, corresponding to the spacing of (200) planes of the fcc Cu2O (the insets in Figs. 3A and 3B are the corresponding original HRTEM derived positions). The well-crystallized nature of both Ag NWs and Cu2O nanocrystals indicates that these Cu2O nanoparticles grew on the surface of Ag nanowires to form heterojunction structures with good contact, which are beneficial for the photoexcited electron transfer between them, and directly contributing to a satisfying photocatalytic performance. The elemental mappings of Ag, Cu and O were shown in Figs. 3D and 3E, it can be found clearly that Ag element lies in the center of the two kinds of Ag and Cu2O composites. Comparing the Cu and O element distributions in both of the samples, the difference can be easily observed. For Ag-Cu2O nanocorncobs, the Cu2O nanoparticles selectively loaded on the surface of Ag nanowires (especially Cu element distribution in Fig. 3C). However, for

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Ag@Cu2O nanocables, both Cu and O elements are uniformly distributed on the surfaces of the Ag nanowires. The results of microscopic images and elemental distribution mappings imply that the size and combining form of Cu2O could be designed and well-controlled by adjusting synthetic parameters. 3.2 The possible formation mechanism In order to well investigate the possible formation mechanism of Ag-Cu2O nanocorncobs, a series of experiments were designed by adjusting (i) the concentrations of surfactant (PVP) and the species of surfactant, (ii) the types of reductant and (iii) the addition amount of NaOH. The detailed experimental parameters were shown in Table 1 and the corresponding SEM images of the obtained samples were also displayed in Fig. 4. Firstly, the effect of the concentration of PVP on the final Ag-Cu2O nanocorncobs morphology was examined. Figs. 4A-4C show that the size of Cu2O nanoislands decreases with the surface-capping group increasing; it is because that the capping agent (PVP) can block the Cu2O further growth. As we know, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) have also been successfully used to control the Cu2O particle morphology and size distribution [25-26]. In this study, these two surfactants were also used to prepare Ag and Cu2O compounds. Unfortunately, no corncob-like Ag-Cu2O nanostructure was found. From Fig. 4D, it can be seen that only separated Ag NWs and Cu2O nanoparticles mixtures were obtained when CTAB was used as surfactant instead of PVP under the same synthesis conditions. Similarly, when SDS was used as surfactant instead of PVP, Ag NWs and fiber-like Cu2O mixtures were observed (Fig. 4E). These results indicate that the size and morphology of Cu2O strongly depend on the surfactant. With a suitable surfactant of PVP, uniform Cu2O nanoislands could grow heteroepitaxially on the surface of Ag NWs to form corncob-like nanocomposites. In 12

addition, it was found that the Cu2O nanoislands were also affected by the polymerization degree of PVP. With PVP-K30 with a low average molecular weight of 4000, although the final synthesized Cu2O could grow on the surface of Ag NWs, the size of Cu2O particle is not uniform (Fig. 4F). It can be conclude that the PVP macromolecules with a high average molecular weight (Mw ≈ 36,0000) adsorbed on the surface of the initial Cu2O nanocrystal core may control easily its growth and efficiently prevent the agglomeration of nanoparticles. Secondly, besides glucose, hydroxylamine hydrochloride (NH2OH∙HCl) and hydrazine (N2H4) were also employed as reductants for the synthesis of Ag and Cu2O composites at room temperature [27-28]. As shown in Fig. 4G, when NH2OH∙HCl was used as a reductant instead of glucose, some of Ag@Cu2O core-shell nanowires were observed. However, most of Ag NWs remained clean and smooth without Cu2O coated. In addition, when NH2OH∙HCl was added into the as-prepared Ag NWs solution, it was also found that some atropurpureus precipitate of AgCl appeared immediately. When a strong reductant N2H4 was used, even with PVP as an effective surfactant, most of Cu2+ ions were reduced to form favorably the self-assembled Cu2O octahedra (Fig. 4H). These results indicate that the final morphology of Ag-Cu2O composites was also affected largely by the performance of the reduction agent. A mild glucose reductant is more appropriate for the successful formation of the designed Ag-Cu2O nanocorncobs. In addition to capping agent and reductants, the role of NaOH was also discussed. Fig. 4I shows that the reduced Cu2O nanoparticles could also be coated partially on the surface of Ag NWs without the use of NaOH, but the composites are nonuniform. Therefore, the addition of NaOH affected the ability of glucose to reduce Cu2+ ions during the formation of Ag-Cu2O nanocorncobs. Considering the above experimental results and referencing a similar report on the

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preparation of Cu2O [29], the involved reactions for the reduction of Cu2+ ions by glucose in our experiment conditions may be described as the following equations: Cu2+ + 2OH─ → Cu(OH)2 ↓

(2)

Cu(OH)2 + 2OH─ → (Cu(OH)4)2─

(3)

2(Cu(OH)4)2─ + C6H12O6 → Cu2O↓ + C6H11O7─ + 3OH─ + 3H2O

(4)

Finally, considering our previous report on the formation of Cu2O nanomaterials [30], the effect of solvent was also studied. When ethylene glycol (EG) was used as a solvent instead of water, the smaller Cu2O nanoparticles were loaded on the surface of Ag NWs at the same reaction conditions as the experiment “A” in Table 1, as shown in Fig 4J. This result suggests that the reduction reaction of Cu2+ ions become slower in the EG than in H2O. Without the addition of PVP (Fig 4K), it is surprised that very little Cu2O nanoparticles grew on the surface of Ag NWs, maybe most of Cu2O nanoparticles were separated from Ag NWs. This result indicates that PVP in synthesis process acted as not only a surfactant but also an effective binder which might contribute to the formation of Cu2O on the surface of Ag NWs. It is interesting that Ag@Cu2O nanocables could be obtained using EG as a solvent without the addition of NaOH (Fig. 4L). According to the experimental results above, various nano-composite photocatalysts composed of Ag and Cu2O could be prepared controllably by adjusting the synthetic parameters. A corncob-like heterostructured photocatalyst of Ag NWs and Cu2O nanoislands composites has been successfully designed and prepared. 3.3 Photocatalytic performances

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To verify the advantage of Ag NWs in photocatalysis, pure Cu2O NWs (with wide ~180 nm) were also prepared. The same amount (0.1 g) of pure Cu2O, Ag-Cu2O and Ag@Cu2O samples was used for UV-vis adsorption test. As shown in Fig. 5A, it is understandable that pure Cu2O NWs have a strongest Uv-vis absorption during 350-550 nm due to the most amount of Cu2O among three samples. Fig. 5A shows that the absorption edges of Ag@Cu2O nanocables and AgCu2O nanocorncobs have an obvious blue-shift compared with that of pure Cu2O NWs. Additionally, it is noteworthy that Ag@Cu2O nanocables has stronger adsorption intensity than Ag-Cu2O nanocorncobs. As the amounts of Cu2O in nanocables and nanocorncobs close to the same, the enhanced adsorption intensity for Ag@Cu2O is largely attributed to the whole cover of Cu2O shell. To well investigate the effects of Ag NWs on the electronic transport, the photocurrents of the composites and pure Cu2O NWs were measured (Fig. 5B). Apparently, it is found that Ag-Cu2O nanocorncobs and Ag@Cu2O nanocables exhibited a noticeably improved photocurrent density compared with the pure Cu2O NWs, which achieved a fivefold and twofold increase, respectively. The enhanced photocurrent density for composites is attributed to that Ag NWs could act as a conductor wire core to effectively accelerate the transfer of electrons which were generated by the p-type Cu2O under visible light irradiation. More importantly, Ag-Cu2O nanocorncobs have a better electron transfer performance and higher photoelectrochemical activity than Ag@Cu2O nanocables due to the fact that the Cu2O nanoislands non-densely loaded on the surface of Ag NWs resulted in some exposed Ag surface which directly contacted with the reaction solution. In another words, the exposed Ag surface is favourable to the electronic output from the catalyst to the external circuit. In order to test and verify the photocatalytic activity of the synthesized composites, the degradation of MO in aqueous solution was performed under visible light. From Fig. 5C, it can be seen that both the as-synthesized Ag@Cu2O and Ag-Cu2O

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composites exhibited more excellent photocatalytic activity than pure Cu2O NWs. The corncoblike Ag-Cu2O indeed had the highest photocatalytic activity; nearly 86% of MO was degraded in 150 min under visible light irradiation, which is significantly higher than those of Ag@Cu2O (67%) and pure Cu2O (50%). These results demonstrate that Cu2O nanocrystal combined with Ag NWs could effectively enhance the photocatalytic performance, and the structural morphology of heterostructures further affected the photocatalytic property. For corncob-like Ag-Cu2O, the highly improved photocatalytic activity is attributed to an excellent electronic conductivity of Ag and suitably exposed Ag surface contacted with the reaction media. Based on the fact of the higher photocatalytic activity of Ag-Cu2O nanocorncobs than both Ag@Cu2O nanocables and pure Cu2O NWs, a schematic illustration of charge transfer in the AgCu2O system is shown in Fig. 5D. Since the bottom of the CB of Cu2O is higher than the Fermi energy level of Ag, the photogenerated electrons on Cu2O can transfer from the CB of Cu2O to the Ag NWs [31]. For the corncob-like Ag-Cu2O composite photocatalyst, the electrons on the surface of Ag NWs can directly connected with the surrounding solution through the exposed Ag surfaces, and these electrons can quickly participate in the multiple-electron reduction reaction of oxygen (in the reaction solution) to form the superoxide anion radicals (∙O2-) and hydroxide radicals (∙OH), which can efficiently oxidize organic molecules [32-34]. Therefore, the photocorrosion reaction (1) of Cu2O reduced to Cu by electron can be effectively restrained. At the same time, the holes on the valence band (VB) of Cu2O are energetic and can degrade organic pollutants like MO [35-37]. As a result, the novel corncob-like Ag-Cu2O composites with suitably exposed Ag surface not only effectively suppress the photocorrosion of Cu2O and inhibit the recombination of electron-hole pairs but also suitably increase the active sites of electronic

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conduction and accelerate the oxidation of organic molecules. Therefore, Ag-Cu2O nanocorncobs exhibit much higher catalytic activity than Ag@Cu2O nanocables and pure Cu2O NWs. 4. Conclusions In summary, two kinds of heterostructures of Ag-Cu2O nanocorncobs and Ag@Cu2O nanocables have been synthesized by adjusting synthetic parameters. The Ag core nanowires as an excellent conductor effectively promoted the separation of photogenerated electron-hole pairs on the Cu2O, and thus increasing the photocatalytic activity. Especially, the corncob-like AgCu2O nanostructure with suitably exposed Ag surface exhibited much higher photocatalytic activity than Ag@Cu2O nanocables and pure Cu2O NWs under visible light irradiation.

Acknowledgements This work was supported by the finanical support from the National Natural Science Foundation of China (No. 20873044) and the Guangdong Provincial Natural Science Foundation of China (No. 2014A030312007).

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Figure Captions Fig. 1 (A) XRD patterns of Ag@Cu2O nanocables and Ag-Cu2O nanocorncobs. (B) EDX spectrum of Ag-Cu2O nanocorncobs. (C) A standardless atom counting results from (B) Fig. 2 Typical large-scale SEM images for Ag NWs (A), Ag-Cu2O nanocorncobs (B) and Ag@Cu2O nanocables (C). High magnification SEM images for Ag-Cu2O nanocorncobs (D) and Ag@Cu2O nanocables (E). Typical TEM images for Ag-Cu2O nanocorncobs (F) and Ag@Cu2O nanocables (G). Fig. 3 HRTEM images for Ag-Cu2O nanocorncobs (A) and Ag@Cu2O nanocables (B). TEM images and the corresponding elemental (contain Ag, Cu and O) mappings of Ag-Cu2O nanocorncobs (C) and Ag@Cu2O nanocables (D). Fig. 4 SEM images of the as-prepared Ag-Cu2O composites under different experiment conditions corresponding to Table 1. Fig. 5 (A) UV-vis spectra of Ag@Cu2O, Ag-Cu2O and pure Cu2O NWs. (B) Transient photocurrent-time profiles at a bias of -0.1 V versus SCE for Ag@Cu2O nanocables, Ag-Cu2O nanocorncobs and pure Cu2O NWs under visible light irradiation (λ> 420 nm). (C) Photocatalytic degradation of MO on Cu2O NWs, Ag@Cu2O, Ag-Cu2O and none catalyst under visible light irradiation. (D) Charge transfer diagram in Ag-Cu2O heterostructure. Here, Ef is Fermi energy level, CB is conduction band, and VB is valence band.

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Table 1 Experimental parameters for producing Ag NWs and Cu2O nanocomposites

a

Surfactant

Reductant a

NaOH

Sample No.

Reaction solution (40 ml)

A

H2O

PVP (0.8)

glucose

2.5

B

H2O

PVP (1.0)

glucose

2.5

C

H2O

PVP (1.2)

glucose

2.5

D

H2O

CTAB (0.8)

glucose

2.5

E

H2O

SDS (0.8)

glucose

2.5

F

H2O

PVP K30(0.8)

glucose

2.5

G

H2O

PVP (0.8)

NH2OH∙HCl

2.5

H

H2O

PVP (0.8)

N2H4

2.5

I

H2O

PVP (0.8)

glucose

0

J

EG

PVP (0.8)

glucose

2.5

K

EG

0

glucose

2.5

L

EG

PVP(0.8g)

glucose

0

(g)

(ml)

The volume of glucose is 5 ml (0.36 M), the volume of NH2OH∙HCl is 2 ml (0.2 M) and the volume of N2H4

is 40 µl. The reaction temperatures are 80 oC for A-F and H, 100 oC for J-L and 25 oC for G and I. The reactions were terminated by the solution color remarkable change to light yellow.

22

Fig. 1.

23

Fig. 2.

24

Fig. 3.

25

Fig. 4.

26

Fig. 5.

27