β-Ni(OH)2 flower microspheres with enhanced photocatalytic performance

Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 125–130

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One-pot synthesis of Ag/␤-Ni(OH)2 flower microspheres with enhanced photocatalytic performance You-Cun Chen a,b,∗, Fang-Cai Zheng a,b, Yu-Lin Min a,b, Tao Wang a,b, Yan-xia Wang a,b, Yuan-Guang Zhang a,b a b

School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, PR China Anhui Provincial Laboratory of Optoelectronic and Magnetism Functional Materials, Anqing Normal University, Anqing 246011, PR China

a r t i c l e

i n f o

Article history: Received 31 August 2011 Received in revised form 31 October 2011 Accepted 7 December 2011 Available online 16 December 2011 Keywords: Flower microspheres Composite materials Photocatalytic activity Hydrothermal method

a b s t r a c t In this article, three-dimensional (3D) Ag/␤-Ni(OH)2 flower microspheres were synthesized through a facile hydrothermal method in the presence of l-arginine. The results suggest that this biomoleculeassisted hydrothermal method is an efficient route for the fabrication of Ag/␤-Ni(OH)2 flower microspheres by using l-arginine as both a shape controller and a reducing agent of silver ions. The assynthesized products were characterized by X-ray power diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The photocatalytical activity of the Ag/␤-Ni(OH)2 flower microspheres was also investigated by UV–Vis spectrophotometer. And the results display enhanced photocatalytic activity in degradation of methylene blue. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Transitional metal hydroxides, such as iron, cobalt, and nickel, have attracted much attention owning to their much potential application in a variety of fields [1–6]. As a typical metal hydroxide, nickel hydroxide (Ni(OH)2 ) has been widely investigated because of its applications in nickel-based rechargeable alkaline batteries [7–10]. The performance of nickel hydroxide cathodes is directly affected by their size and morphologies [11,12]. It was reported that nickel hydroxide electrodes could be significantly enhanced when nanophase nickel hydroxide was added to micrometer-size spherical nickel hydroxide [13]. Therefore, fabrication of nickel hydroxide with nanostructure is of crucial importance in highenergy-density batteries. Recently, Ni(OH)2 microspheres has been successfully prepared by the assistance of templates [14]. However, the as-prepared Ni(OH)2 samples have poor structure and stability with this method. Moreover, the template-assisted process is very complicated and removing templates is time-consuming [15]. To date, morphology-controlled synthesis and large-scale selfassembly of the nanoscale building blocks into curved structure,

∗ Corresponding author at: School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, PR China. Tel.: +86 556 5500090; fax: +86 556 5500090. E-mail address: [email protected] (Y.-C. Chen). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.016

such as nano/microflower and nanosheet, have been a big challenge for materials synthesis because of their high potential in new technological applications. These curved structures are benefit to synthesize hybrid materials because the structures could allow the platform to accommodate a secondary material like noble metal nanoparticals. Recently, Tarasankar group has successfully synthesized Ag/Ni(OH)2 flower-like nanocomposites with two steps [16]. At first, the Ni(OH)2 precursor was synthesized by hydrothermal route. Secondly, the Ag/Ni(OH)2 nanocomposites were obtained through the reduction of Ag+ in the Ni(OH)2 precursor suspension under UV-light irradiation. This method for preparing the Ag/Ni(OH)2 nanocomposites was very complicated, and most importantly, the reduction process often leads metal self-nucleation and produces the unexpected isolated metal nanoparticles. In order to overcome these shortcomings, we concentrate on one-step green method to prepare Ag/Ni(OH)2 nanocomposites. Recently, we succeeded in synthesizing flower-like Ag/␤-Ni(OH)2 microspheres by the hydrothermal synthesis of AgNO3 and Ni(AC)2 at low temperature in the presence of l-arginine. Compared with many reports on the synthesis of Ag/Ni(OH)2 nanocomposites, this template-free approach simplifies synthesis procedure, shortens reaction time, reduces reaction temperature and is a green synthetic route. Moreover, this one-pot synthesis could be potentially used to prepare other nanocomposites. And then the Ag/Ni(OH)2 flower microspheres could display excellent catalytic activity.

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2.3. Photocatalytic test The photocatalytic degradation of methylene blue was performed under an ambient atmosphere in a reactor. An aliquot of 50 mL of an aqueous solution of methylene blue (10−7 M) was taken in 100 mL vitreous round-bottom flasks with certain amount of catalyst in each flask. The system was allowed to stand for 20 min in the dark to ensure establishment of adsorption equilibrium of the dye on the catalyst surface, and then was irradiated under UV light from the top. The radiating source is a 100 W Philips medium-pressure mercury lamp ( = 365 nm). At identical time intervals, 5 ml samples of the suspension were withdrawn and were immediately centrifuged and then filtered to remove completely any catalyst particle, and methylene blue concentrations were analyzed using a UV–Vis spectrophotometer at a wave length of 664 nm. 2.4. Characterization The as-prepared samples were determined by X-ray power diffraction (XRD)(Shimadzu Corporation, Japan) on an X-ray diffractionmeter with Cu K␣ radiation ( = 0.15418 nm) at 2 ranging from 10◦ to 80◦ . The morphologies of the as-obtained samples were examined with the JSM-6700F scanning electron microscope (SEM), and the transmission electron microscope (TEM) and the energy-dispersive spectrometry (EDS) result was performed on a JEOL 2010 high-resolution at 200 kV. UV–Vis spectra were recorded on a Shimadzu UV-240 spectrophotometer at room temperature. 3. Results and discussion Metallic Ag modified 3D ␤-Ni(OH)2 flower microsphers were synthesized through a facile one-pot hydrothermal method as described in Section 2. The as-prepared products were characterized by X-ray power diffraction (XRD). Fig. 1(b) shows the XRD pattern of Ag/␤-Ni(OH)2 composite. The diffraction perks confirm the sample as a mixture of face-centered cubic lattice of Ag with cell constants of a = 4.086 A´˚ (JCPDS Card File No. 04-0783) and the single

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2.2. Synthesis of ˇ-Ni(OH)2 and Ag/ˇ-Ni(OH)2 flower microspheres In a typical synthesis, 12 ml of aqueous solution containing 1 mmol (0.2487 g) nickel acetate dihydrate and 3 mmol (0.5226 g) l-arginine, and 400 ␮L 0.02 M (0.008 mmol) sliver nitrate solution, and 6 ml of absolute ethanol were mixed with agitation in a beaker. Then, 1 ml of 28 wt.% ammonium aqueous solution was added dropwise to the above mixed solution. After being vigorously stirred for ten minutes, the solution became blue, and then the resulting solution was put into a 25 ml Teflon-lined stainless steel autoclave and heated at 100 ◦ C for 8 h. And the ␤-Ni(OH)2 flower microspheres were prepared via a similar procedure without sliver nitrate. After reaction, the autoclaves were allowed to cool naturally. The precipitates were collected, washed with distilled water and absolute ethanol several times, and finally dried in a vacuum for 6 h.

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All the reagents were purchased from Shanghai Reagent Co. of Chinese Medical, including nickel acetate dihydrate (Ni(AC)2 ·2H2 O), 28 wt.% ammonium aqueous solution, silver nitrate (AgNO3 ), methylene blue, l-arginine (C6 H14 N4 O2 ) and ethanol. They were analytical grade and used without further purification.

*N i(OH ) 2 #A g

Intensity (a.u.)

2.1. Materials

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2 Thata (degree) Fig. 1. XRD patterns of ␤-Ni(OH)2 flower microspheres and Ag/␤-Ni(OH)2 composites. (a) ␤-Ni(OH)2 and (b) Ag/␤-Ni(OH)2 (*␤-Ni(OH)2 ;#Ag).

phase of ␤-Ni(OH)2 with a suitably crystalline hexagonal struc´˚ c = 4.605 A, ´˚ JCPDS Card File No. 14-0117). Fig. 1(a) ture (a = 3.126 A, shows the XRD pattern of pure ␤-Ni(OH)2 flower microspheres which were prepared via a similar procedure without AgNO3 in the reaction solution. Compared with the diffraction peaks of the pure ␤-Ni(OH)2 , no characteristic peaks of impurities and other phases such as NiO and Ag2 O are observed. In addition, negligible changes of all diffraction peak positions of ␤-Ni(OH)2 in all Ag/␤-Ni(OH)2 samples compared to that of pure ␤-Ni(OH)2 suggest that Ag does not incorporate into the lattice of ␤-Ni(OH)2 , but as metal deposit on the surface. The ␤-Ni(OH)2 products were obtained by a facile one-step chemical process for 8 h at 100 ◦ C. The morphology of the as-made products can be illustrated by the field emission scanning electron microscope (FESEM). The FESEM results (Fig. 2(a–c)) showed that the as-prepared ␤-Ni(OH)2 product was mainly composed of large quantity of flowerlike microspheres and the diameter of the samples was uniform in size about 2–3 ␮m. As shown in the high-magnification FESEM images in Fig. 2(b) and (c), the 3D flower microspheres consisted of nanoslices petal with thickness of 20–30 nm in size. The morphology of the Ag/␤-Ni(OH)2 composites are also shown in Fig. 2(d–f). It is obvious seen that the main morphology of the Ag/␤-Ni(OH)2 remains flower microsphere structure which is similar with that of the ␤-Ni(OH)2 sample. After adding AgNO3 to the reaction solution, the morphology of ␤-Ni(OH)2 flower microspheres had no change and the Ag particles could be highly dispersed on the surfaces of ␤-Ni(OH)2 flower microspheres. And the diameter of Ag particle is about 50–200 nm. From Fig. 2(c) and (f), it is clearly seen that the slippery surface of the petal of ␤Ni(OH)2 flower microspheres have changed to coarse surface of the Ag/␤-Ni(OH)2 composites. In addition, as shown in Fig. 2(f), there are a quantity of Ag particles deposited on the surface of the petal of ␤-Ni(OH)2 flower microspheres. The ␤-Ni(OH)2 flower microspheres used as matrix for the synthesis of hybrid nanomaterials were prepared by a facile one-pot hydrothermal method. Fig. 3(a) shows a typical TEM image of the single Ag/␤-Ni(OH)2 microspheres. From this image, the regular flower-like morphology and diameter of 2–3 ␮m can be observed. As observed, Ag particles with diameter of 50–200 nm are deposited on the surface of the petal of ␤-Ni(OH)2 flower microspheres. No isolated Ag particles exists in the sample except for the surface of the ␤-Ni(OH)2 microspheres because of the strong electrostatic attraction between negatively charged microspheres and positively charged Ag+ [17].

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Fig. 2. (a–c) SEM images of ␤-Ni(OH)2 flower microspheres; (d–f) SEM images of Ag/␤-Ni(OH)2 flower microspheres.

The Ag/␤-Ni(OH)2 composites were further investigated by energy-dispersive spectrometry (EDS). The EDS data in Fig. 3(b) confirmed that the element of the selected area was Ag, Ni and O and without any other impurities. From Fig. 3(c), it is clearly confirmed that the shallow color area contains Ni, O and a few Ag and without any other impurities. The Cu and C peaks come from the Cu grid used in TEM measurements. Moreover, the strong perks for Ni, Ag and O elements in the EDS data (Fig. 3(b)) further confirm the formation of Ag/␤-Ni(OH)2 hybrid nanomaterials. The morphology of the as-made ␤-Ni(OH)2 product with different ratio of nickel acetate and l-arginine was also investigated by FESEM. As shown in Fig. 4(a) and (b), when the ratio of nickel acetate and l-arginine was changed to 1:1, the morphology of the ␤-Ni(OH)2 samples became irregular, and the samples were mixed with flower microspheres and nanosheets. When the ratio of the nickel acetate and l-arginnine was converted to 1:2, as shown in Fig. 4(c) and (d), the morphology of the ␤-Ni(OH)2 samples still revealed the flower microspheres. Comparing to Fig. 2(c), however, the petals of the ␤-Ni(OH)2 samples were irregular and

the thickness of the nanoslices decreased to about 10 nm from 20–30 nm. In addition, the influence of the mass of l-arginine to the Ag/␤-Ni(OH)2 composites has also investigated when keeping the same as described in Section 2. When the mass of l-arginine was changed to 2 mmol, the Ag/␤-Ni(OH)2 composites were obtained with 400 ␮L 0.02 M AgNO3 solution added into the reactive solution, as shown in Fig. 4(e) and (f). The Ag/␤-Ni(OH)2 samples still showed the flower microspheres, but a few Ag nanoparticles deposited on the surface of the petal of the as-prepared sample, as marked in Fig. 4(f). The mass of l-arginine could influence the content of Ag deposited on ␤-Ni(OH)2 , because the l-arginine has enough ability to reduce metallic ions into metal. When the mass decreases, the ability is too weak to reduce enough metallic ions into metal. One the basis of the above analysis, the probable formation mechanism of the flowerlike Ag/␤-Ni(OH)2 composites have been proposed. At the initial stage of the reaction, Ni2+ and Ag+ coordinate with l-arginine which has two kinds of functional groups (−NH2 and −COOH) [18]. Furthermore, Ni(NH3 )6 2+ and Ag(NH3 )2 +

Fig. 3. (a) TEM image of the as-synthesized Ag/␤-Ni(OH)2 composites; (b and c) EDS spectra corresponding to Ag/␤-Ni(OH)2 flower microspheres.

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Fig. 4. (a) and (b) SEM images of ␤-Ni(OH)2 flower spheres (MNickel acetate :Ml-arginine = 1:1); (c) and (d) SEM images of ␤-Ni(OH)2 flower spheres (MNickel acetate :Ml-arginine = 1:2); (e) and (f) SEM images of Ag/␤-Ni(OH)2 flower spheres (MNickel acetate :Ml-arginine = 1:2; 0.4 ml 0.02 mol/L AgNO3 ).

complexes were formed in the presence of ammonia group respectively, as shown by the following equations [19]: Ni2+ + 6NH3 → Ni(NH3 )6 2+ +

Ag + 2NH3 → Ag(NH3 )2

+

(1) (2)

With reaction time prolonging, flowerlike Ni(OH)2 slowly formed. At the same time, l-arginine which could reduce metallic ions into metal plays dual and crucial roles in attaching noble metals with uniform size and distribution onto Ni(OH)2 microspheres. Fig. 5 shows the probable schematic diagram for the growth process of the Ni(OH)2 based composite nanomaterials. The −NH2 is able to bind Ag nanoparticles, thus making Ag nanoparticles stable from aggregation. In addition, both amino and carboxylic groups have been reported to be capable of coordinating with transition-metal oxides whose surfaces are negatively charge [18]. The charge of transition-metal hydroxides surface is similar to transition-metal oxides. It suggests that an amino acid can serve as a bridge or linker to anchor noble metal onto hydroxides to form

noble metal/dyhroxides composite nanomaterials. The l-argininefunctionalized Ni(OH)2 nanoparticles can absorb metallic Ag due to the strong interactions between amino groups and metal particles. Therefore, the overall chemical reactions can be formulated as follows [20,21]: NH3 + H2 O → NH4 + + OH− Ni(NH3 )6

2+

−

+ 2OH → Ni(OH)2 ↓ + 6NH3

L-arginine Ni(OH)2 + Ag(NH3 )+ −→ Ag/Ni(OH)2 2

(3) (4) (5)

In order to evaluate the photocatalytic activity of the Ag/␤Ni(OH)2 flower microspheres, the photocatalytic degradation rate of methylene blue has been measured in the presence of the Ag/␤-Ni(OH)2 composites. The initial concentration of catalyst on degradation of methylene blue was investigated with 50 mL of an aqueous solution of methylene blue (10−7 M). As shown in Fig. 6, when the initial concentration of catalyst was lower than 0.16 mg/50 ml, the degradation rate of methylene blue was

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Fig. 5. The probable formation mechanism of the Ag/(-Ni(OH)2 composites in the presence of l-arginine.

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enhanced. However, the degradation rate of methylene blue decreased when the initial concentration of catalyst was higher than 0.16 mg/50 ml. It was concluded that too little or too much catalyst could weaken the degradation rate of methylene blue [22,23]. As photocatalytic reference, the pure ␤-Ni(OH)2 was used to evaluate the activity of the Ag/␤-Ni(OH)2 sample. From Fig. 7, it is clearly observed that the Ag/␤-Ni(OH)2 microspheres have shown better photocatalytic performance than the ␤-Ni(OH)2 microspheres. It has been reported that the photocatalytic performance of ZnO samples can be significantly improved by depositing an appropriate amount of Ag nanoparticles [21]. In this experiment, the Ag/(-Ni(OH)2 microspheres have uniform Ag particles and high surface of (-Ni(OH)2 which can effectively catalyze the organism that mainly contains azo-type structure [24].The activity of the Ag/␤-Ni(OH)2 nanocatalyst is dependent on many factors, such as the structure of catalysts, content of Ag and predominant chemical state of Ag [25]. The Ag nanoparticles on the surface of the ␤-Ni(OH)2 micropheres act as a sink for the electrons, promote interfacial charge-transfer kinetics between the metal and the semiconductor, improve the separation of photogenerated electron–hole pairs, and thus enhance the photocatalytic activity of the Ag/␤-Ni(OH)2 photocatalyst [26]. Therefore, the higher the dispersity of Ag clusters and/or nanoparticles on the surface of ␤-Ni(OH)2 is, the higher the photocatalytic activity of the Ag/␤-Ni(OH)2 photocatalyst should be. The photocatalytic-activity measurements for Ag decorated

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Time/min Fig. 7. Time dependent photocatalytic degradation rate of methylene blue in the presence of (-Ni(OH)2 and Ag/(-Ni(OH)2 microspheres, respectively.

␤-Ni(OH)2 microspheres found that Ag transfers electrons more rapidly and better dissipates the accumulated charge than the ␤-Ni(OH)2 microspheres [27]. It is concluded that Ag nanoparticles loading on the surface of ␤-Ni(OH)2 microspheres must be redounded to generate the electrons and holes to enhance photocatalytic activity.

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4. Conclusions

0.08 mg/50 ml 0.12 mg/50 ml 0.24 mg/50 ml 0.20 mg/50 ml 0.16 mg/50 ml

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In summary, the Ag/␤-Ni(OH)2 composites have been synthesized through biomolecule-assisted hydrothermal method, which is a green synthetic route. The investigation of photocatalytic ability indicated that the Ag/␤-Ni(OH)2 composites possessed higher photocatalytic activity than the pure ␤-Ni(OH)2 microspheres for the degradation of methylene blue under UV light irradiation due to the enhanced separation efficiency of photogenerated electron–hole pairs. These results show that this green method can be an effectively extended to design and fabricate novel catalytic materials for further application.

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Time / min Fig. 6. The effect of the initial concentration of catalyst on degradation of methylene blue.

The present work was supported by the special funding support from Natural Science Foundation of Anhui Province (Nos. KJ2010ZD07, and KJ2008B172) and the National Science Foundation of China (NSFC) (Grants No. 20871005).

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