ZnNi alloy nanoparticles grown on reduced graphene oxide nanosheets and their magnetic and catalytic properties

Abstract

Reduced graphene oxide (RGO)–ZnNi alloys nanocomposites were synthesized by in situ growth of ZnNi alloy nanocrystals on graphene oxide (GO) accompanied with a chemical co-reduction process. The size and morphology of ZnNi alloy nanoparticles on RGO sheets can be tuned by simply changing initial concentrations of metal ions and molar ratio of Zn²⁺ to Ni²⁺ in the reaction system. The as-synthesized products were characterized by powder X-ray diffraction, energy dispersive X-ray spectroscopy, Raman spectroscopy and transmission electron microscopy. Magnetic measurements reveal that the nanocomposites have ferromagnetic characteristics and show composition dependent magnetic properties. Moreover, the RGO–ZnNi nanocomposites show remarkably enhanced catalytic activity and recycling stabilities toward the reduction of 4-nitrophenol (4-NP), and can be easily re-collected from the reaction system by a magnet. It is believed that the obtained magnetic RGO–ZnNi nanocomposites with excellent catalytic properties provide a new opportunity for the application of graphene-based materials.

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1. Introduction

Metal nanomaterials, as one kind of the most important materials, have received wide recognition due to their unique properties and wide applications in various fields.¹ Especially, bimetallic nanocrystals composed of two different metal elements possess interesting size- and composition-dependent electrical, chemical and optical properties.^2–5 Recently, more and more interest is being endowed into the field of alloy catalysis owing to the enhanced performance compared to their individual components. For example, Y. Huang et al. reported that Pt₃Ni nanocrystals can serve as highly efficient catalysts in oxygen reduction reaction with much better performance and durability than those of the commercial Pt black and commercial Pt/C catalysts.⁶ Since the reduction of aromatic nitro compounds to amines is a very vital process in synthetic organic chemistry and in industrial fabrication of many industrially important products, the reduction of 4-NP into 4-aminophenol (4-AP) by NaBH₄ has been widely employed as a model reaction to quantitatively evaluate the catalytic activity of a variety of metal or alloy catalysts.^7–10 For instance, K. Mandal et al. reported that NiCo alloy nanochains show excellent catalytic activity in the borohydride reduction of 4-NP.¹⁰ These studies provide an important hint that bimetal or alloy nanoparticles would have excellent catalytic performance on the reduction of 4-NP. As for alloys, we are interested in ZnNi alloys, which have been widely used in many fields because of their excellent corrosion resistance.^11-13 Recently, ZnNi alloys are found to be a new type of catalytic materials possessing high catalytic activity.^14,15 The unsupported ZnNi has been used as methanol steam reforming catalysts by Friedrich et al.¹⁴ ZnNi catalysts supported on TiO₂, TiO₂–SiO₂, TiO₂/SBA-15 and SBA-15 substrates have been prepared by Loricera et al. and they show superior catalytic activities in the hydrodeoxygenation of phenol and the hydrogenation of synthetic diesel.¹⁶

Graphene, an atom-thick two-dimensional (2D) carbon material, is emerging as one of the most appealing catalytic support materials due to its unique structure and excellent properties such as superior electrical conductivity, excellent mechanical flexibility, high thermal and chemical stability,¹⁷ extremely high surface area (theoretical value of 2630 m² g⁻¹).¹⁸ The integration of graphene with other materials including metal,¹⁹ semiconductor,²⁰ ceramics,²¹ polymer²² and other carbon materials²³ is becoming a promising approach to harness the favorable properties of graphene for practical applications. It has been demonstrated that graphene-supported FePt,²⁴ FeAu,²⁵ PtAu,²⁶ AgAu,²⁷ and FeNi²⁸ exhibit unusually high catalytic performance, which makes graphene an ideal substitute for other carbon materials as catalyst support. However, to our knowledge, the integration of nano-sized ZnNi with graphene has not been reported so far. Herein, we develop an easy wet chemical route to grow ZnNi nanocrystals on reduced graphene oxide (RGO) nanosheets for the first time. The morphology and size of ZnNi alloy on RGO sheets can be adjusted by simply changing initial metal ions concentrations and molar ratio of Zn²⁺ to Ni²⁺. The graphene-based RGO–ZnNi nanocomposites show ferromagnetic property and excellent catalytic performance toward the reduction of 4-NP.

2. Experimental

2.1. Materials

Natural flake graphite with a particle size of 150 μm (99.9% purity) was purchased from Qingdao Guyu Graphite Co., Ltd. All of the other chemical reagents used in our experiments were of analytical grade, purchased from Sinopharm Chemical Reagent Co., Ltd, and used without further purification. Graphite oxide was prepared from the natural flake graphite according to a modified Hummers method.^29,30

2.2. Preparation of ZnNi alloys on RGO

The typical procedure for the preparation of the alloy of ZnNi₅ nanoparticles attached on RGO nanosheets is as follows: 50 mg of GO was sonicated in 140 mL of ethylene glycol (EG) to form a uniform GO dispersion. Subsequently, 0.84 mmol of Ni(NO₃)₂·6H₂O and 0.17 mmol of Zn(NO₃)₂·6H₂O (with initial molar ratio of Zn : Ni = 1 : 5) were dispersed in 10 mL of EG, and then gradually added into the GO dispersion. The resulting mixture was magnetically stirred at room temperature for 1 h, and heated to 110 °C under N₂ atmosphere. Then, 25 mL of hydrazine hydrate dissolved with 1 g of NaOH was slowly added in, and the resulted mixture was refluxed at 110 °C for 45 min. After the reaction system was naturally cooled to room temperature, the black precipitates were collected by centrifugation, washed with distilled water and absolute ethanol several times, and dried in a vacuum oven at 45 °C for 24 h. For comparison, ZnNi alloy nanoparticles with different sizes and compositions were grown on RGO nanosheets by adjusting the initial concentrations and molar ratios of Zn²⁺ and Ni²⁺ ions. In addition, RGO–Ni, bare ZnNi₅, and pure RGO were also prepared in the same experimental conditions in the absence of Zn(NO₃)₂·6H₂O, GO, and both Zn(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O, respectively. Detailed parameters for these synthetic experiments are shown in Table 1.

Table 1 Samples of RGO–ZnNi nanocomposites synthesized with various initial feeding amounts and molar ratios of Zn²⁺ and Ni²⁺

Samples	Initial feeding amount of metal ions (mmol)	Zn^2+/Ni^2+ molar ratio	ZnNi content^a wt%
a The contents of ZnNi alloys in the samples were determined by ICP-OES.
RGO–ZnNi5-1	n(Zn^2+) + n(Ni^2+) = 1.5	1 : 5	70.87
RGO–ZnNi5-2	n(Zn^2+) + n(Ni^2+) = 1.0	1 : 5	55.73
RGO–ZnNi5-3	n(Zn^2+) + n(Ni^2+) = 0.5	1 : 5	50.03
RGO–ZnNi4	n(Zn^2+) + n(Ni^2+) = 1.0	1 : 4	59.76
RGO–ZnNi3	n(Zn^2+) + n(Ni^2+) = 1.0	1 : 3	62.74
RGO–ZnNi2	n(Zn^2+) + n(Ni^2+) = 1.0	1 : 2	66.17
RGO–Ni	n(Ni^2+) = 1.0	—	57.64
ZnNi5	n(Zn^2+) + n(Ni^2+) = 1.0	1 : 5	100

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2.3. Instrumentation and measurements

The phase structures of the as-synthesized products were characterized using X-ray diffraction (XRD, Bruker D8 ADVANCE) with Cu-Kα radiation (λ = 1.5406 Å) at a scanning rate of 4° min⁻¹. The morphology and the size of the products were examined by field emission scanning electron microscopy (FESEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2100). Samples for TEM were prepared by dropping the products on a carbon-coated copper grid after ultrasonic dispersion in absolute ethanol and allowed to dry in air before analysis. The compositions of the products were determined by energy-dispersive X-ray spectrometry (EDS) and inductively coupled plasma optical emission spectrometry (ICP-OES, Vista-MPX). EDS was recorded with an energy dispersive spectrometer attached to the scanning electron microscope (JSM-6480). Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer with KBr pellets. Raman scattering was performed on a JY-HR800 Raman spectrometer using a 453 nm laser source. The magnetic measurements were performed using a vibrating sample magnetometer (VSM, Nanjing University HH-15) at room temperature (300 K). Ultraviolet-visible (UV-vis) spectroscopy measurements were performed on a UV-2450 ultraviolet-visible spectrophotometer.

2.4. Catalytic reduction of 4-NP

The reduction reaction of 4-NP by NaBH₄ was used as a model system to quantitatively evaluate the catalytic activity of the as-synthesized nanocomposites. In a typical reaction, 5 mg of the as-synthesized samples and 2 mL of freshly prepared NaBH₄ solution (1.5 mM) were added to 100 mL of DI water. After stirred for 15 min at 25 °C, 1 mL of 4-NP solution (5 mM) was injected into the mixture to start the reaction. During the reaction process, 2.0 mL of reaction solution was withdrawn at a given time interval, which was immediately recorded in the UV-vis spectrophotometer in a scanning range of 250–500 nm at 25 °C. For successive recycling of the catalysts, the catalysts were magnetically separated from the solution, and then were added into another freshly prepared mixed solution of 2 mL of NaBH₄ solution and 1.0 mL of 4-NP solution to start the next round of reaction.

3. Results and discussion

3.1. Characterization of RGO–ZnNi

In this study, RGO–ZnNi nanocomposites were prepared through a simple one-pot co-reduction method, in which Zn²⁺, Ni²⁺ and GO were reduced by hydrazine hydrate simultaneously. As we all know, GO sheets are highly negatively charged when dispersed in EG because of the ionization of the carboxylic acid and phenolic hydroxyl groups that exist on their edge and surface.³¹ When positively charged Zn²⁺ and Ni²⁺ ions and negatively charged GO sheets were introduced together, the electrostatic interactions between these components provide a strong driving force for the effective adsorption of Zn²⁺ and Ni²⁺ onto GO sheets. With the addition of hydrazine hydrate, Zn²⁺ and Ni²⁺ ions were in situ reduced into ZnNi alloy on GO surface and GO was reduced into RGO simultaneously, as a result, RGO–ZnNi nanocomposites were formed.

The XRD patterns of the as-prepared samples are shown in Fig 1a. The diffraction pattern of RGO–Ni exhibits three peaks at 2θ = 44.31°, 51.64° and 75.94°, corresponding to the (111), (200) and (220) crystal planes of face-centered cubic (fcc) Ni (JCPDS 01-1260). The XRD patterns of RGO–ZnNi nanocomposites are very similar to that of RGO–Ni, but with a slight shift of peak positions towards lower angles. This is reasonable because the atomic radius of Zn is slightly bigger than that of Ni, and the substitution of Zn for Ni in the Ni-based fcc lattice results in the increase of the lattice constants and the spacings of crystal planes.^32,33 The diffraction peak intensity decreases with the increasing of Zn content, indicating that a higher Zn content in the ZnNi alloys will lead to weaker crystallinity. The characteristic (001) peak at about 10° for graphite oxide disappears in all the diffraction patterns, suggesting that graphite oxide has been flaked in these composites. The characteristic (002) peak of RGO at around 24° is very weak for all the composites, suggesting that the GO is reduced to RGO and the restacking of RGO sheets has been prevented by the metal alloy nanoparticles on them.³⁴ Meanwhile, no zinc and nickel oxides or hydroxides or other impurity phases are detected in these XRD patterns. EDS spectrum of the typical composite of RGO–ZnNi₅-2 is shown in Fig 1b. The detectable elements in RGO–ZnNi₅-2 includes zinc, nickel, carbon and oxygen, and the Zn/Ni molar ratio obtained from the spectrum is close to the initial Zn²⁺/Ni²⁺ molar ratio of 1 : 5. The carbon element would come from the RGO nanosheets, while oxygen mainly from the residual oxygen-containing functional groups on RGO. The actual contents and chemical compositions of the ZnNi alloys in RGO–ZnNi nanocomposites were further determined by ICP-OES. It was found that the actual compositions of the ZnNi alloys in these composites are in agreement with the initial molar ratios of metal ions (Table 1).

Fig. 1 (a) XRD patterns of the as-prepared nanocomposites. (b) EDS spectrum of RGO–ZnNi₅-2 nanocomposites.

Raman spectroscopy has been widely employed to characterize carbon materials and to distinguish between different forms of ordered and disordered carbon as well as to extract physical parameters of relevance.^35,36 Here, Raman spectroscopy was used to characterize the structural properties of graphite oxide and RGO in the RGO–ZnNi nanocomposites. As shown in Fig. 2a, the Raman spectrum of graphite oxide displays two prominent peaks at ca. 1350 and 1604 cm⁻¹, corresponding to the well-documented D and G bands, respectively, while for the sample of RGO–ZnNi₅-2, the G band moves to ca.1580 cm⁻¹, close to the value of the pristine graphite. It is well known that the G band is usually assigned to the E_2g mode observed for sp² carbon domains, while the D band is associated with structural defects and disorders that can break the symmetry and selection rule.³⁷ The intensity ratio of D to G (I_D/I_G) is inversely proportional to the average crystallite size of graphite materials.³⁸ The I_D/I_G ratio varies from 1.11 for graphite oxide to 1.80 for RGO–ZnNi₅-2 nanocomposites. The increased I_D/I_G ratio suggests the conjugated graphene network (sp² carbon) is re-established after chemical reduction; however, the size of the re-established graphene network is usually smaller than the original graphite oxide, which will consequently lead to an increase of I_D/I_G.³⁹ These results reveal that GO in RGO–ZnNi nanocomposites has been well deoxygenated and reduced.

Fig. 2 (a) Raman spectra and (b) FT-IR spectra of graphite oxide and RGO–ZnNi₅-2 nanocomposites.

Fig. 2b shows the FT-IR spectra of graphite oxide and RGO–ZnNi₅-2 nanocomposites. For graphite oxide, the absorption bands at 3405 and 1403 cm⁻¹ can be assigned to the stretching vibration and deformation vibration of O–H, respectively. The C=O stretching vibration of the COOH groups situated at the edges of graphene oxide sheets is found at 1727 cm⁻¹.⁴⁰ The bands at 1224 and 1049 cm⁻¹ correspond to the stretching vibration peaks of C–O (epoxy) and C–O (alkoxy), respectively. The band at 1621 cm⁻¹ derives from the vibrations of the adsorbed water molecules and also the contributions of the skeletal vibrations of unoxidized graphitic domains.⁴¹ For the RGO–ZnNi₅-2 nanocomposites, most bands that related with the oxygen-containing functional groups vanish in the FT-IR spectrum except a weak peak at approximately 1200 cm⁻¹ (ν(C–O)),⁴² revealing that the GO was effectively deoxygenated and reduced during the reduction process. The new absorption band at 1560 cm⁻¹ can be assigned to the skeletal vibration of the graphene sheets.

To investigate the morphology and microstructure of ZnNi alloys supported on RGO, TEM characterization was performed for several typical samples. Fig 3 shows the TEM images of RGO–ZnNi₅-2 at different magnifications. RGO sheets can be clearly distinguished from the background and are nearly smooth with a few wrinkles, which is well consistent with the bare RGO synthesized in the absence of Zn(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O (Fig. S1†), and the size of the bare RGO nanosheets can be seen to be as large as several micrometers (Fig. S1†). The ZnNi₅ alloy nanoparticles are homogeneously anchored onto the surface of the RGO sheets and are well separated from each other although some of them tend to assemble into chain-like structure. As shown in Fig. 3b, the mean size of the ZnNi₅ nanoparticles is 51.7 nm (Fig. 4d). The good distribution of ZnNi₅ alloy nanoparticles on graphene sheets guarantees the efficient catalytic properties of RGO–ZnNi₅ alloy nanomaterials. Fig. 3c shows a typical individual ZnNi₅ nanoparticle. HRTEM image of the ZnNi₅ nanoparticle is shown in Fig. 3d, which reveals the polycrystalline characteristic of the alloy nanoparticle. The lattice spacing of 0.206 nm for (111) plane of ZnNi₅ is larger than that (0.203 nm) of pure Ni,²⁸ which is consistent with the result of XRD (Fig. 1). For comparison, TEM image of the bare ZnNi₅ prepared in the absence of GO is shown in Fig. 4g. The product consists of ZnNi₅ microspheres with an average diameter of 618.1 nm (Fig. 4h), which is much bigger than the ZnNi₅ nanoparticles in RGO–ZnNi₅-2, exhibiting that GO nanosheets have a significant influence on the shape and size of the ZnNi alloy nanoparticles. This further confirms that GO nanosheets can play an important role in the nucleation and growth of inorganic nanocrystals.^23,43

Fig. 3 (a–c) TEM and (d) HRTEM images of the as-prepared RGO–ZnNi₅-2.

Fig. 4 TEM images and the ZnNi particle size distribution of the (a and b) RGO–ZnNi₅-1, (c and d) RGO–ZnNi₅-2, (e and f) RGO–ZnNi₅-3 and (g and h) the bare ZnNi₅.

In addition, it was found that the size and morphology of the ZnNi alloy nanocrystals on RGO nanosheets strongly depend on the molar ratio of Zn²⁺ to Ni²⁺ (keeping the total mole of metal ions constant: n (Zn²⁺) + n (Ni²⁺) = 1.0 mmol). As shown in Fig. 5, the size and morphology of the ZnNi nanoparticles show a significant change with the increasing content of zinc. The average diameters of the alloy nanoparticles for RGO–Ni, RGO–ZnNi₄, RGO–ZnNi₃ and RGO–ZnNi₂ are 36.7 (Fig. 5c), 75.9 (Fig. 5f), 127.8 (Fig. 5i) and 132.7 nm (Fig. 5l), respectively. The diameter increases with the increasing of Zn/Ni atomic ratio. These nanoparticles are all made up of smaller nanocrystals. Meanwhile, with different Zn/Ni ratio, the morphologies of the ZnNi alloy nanoparticles are also slightly different. The pure Ni nanoparticles exhibit irregular shape, while the ZnNi alloy nanoparticles show regular spherical shape structure, the surface of which becomes smoother with the increasing of Zn content. This result is consistent with that observed in the NiCo alloy nanoparticles.⁴³ With the increasing of Ni content, the ZnNi alloy nanoparticles will be affected more markedly by the magnetocrystalline anisotropy,³³ which results in the coarse surface or even irregular shape of them.

Fig. 5 TEM images and ZnNi particle size distributions of (a–c) RGO–Ni, (d–f) RGO–ZnNi₄, (g–i) RGO–ZnNi₃ and (j–l) RGO–ZnNi₂.

Moreover, the size of the ZnNi nanoparticles on RGO sheets can also be affected by the amount of metal ions used. As shown in Table 1, the samples of RGO–ZnNi₅-1, RGO–ZnNi₅-2 and RGO–ZnNi₅-3 were synthesized with the same Zn/Ni ion ratio but different amounts. The corresponding TEM images and the detailed particle size distribution of the ZnNi particle are shown in Fig. 4. It can be clearly seen that ZnNi nanoparticles are well loaded on RGO nanosheets in all these samples. The average sizes of ZnNi particles in RGO–ZnNi₅-1, RGO–ZnNi₅-2 and RGO–ZnNi₅-3 are 159.9 (Fig. 4b), 51.7 (Fig. 4d) and 28.9 nm (Fig.4f), respectively, revealing that the less dosage of metal ions will lead to the smaller size of ZnNi₅ nanoparticles, which is also consistent with our previous report.²⁸ Also, the distribution density of ZnNi₅ nanoparticles on RGO sheets shows an obvious decrease with the decreasing amount of metal ions.

3.2. Magnetic properties of RGO–ZnNi nanocomposites

Magnetic properties of the as-prepared nanocomposites were investigated at room temperature (300 K) using a vibrating sample magnetometer with an applied field −10 000 Oe ≤ H ≤ 10 000 Oe. For quantitative comparison of our data, we normalized the measured magnetization data with respect to the ZnNi contents in the samples determined by ICP-OES. Thus all our magnetization data are given in the unit of emu per gram of ZnNi alloys. All of the samples exhibit typical ferromagnetic curves (Fig. 6), the values of saturation magnetization (M_s), remanence (M_r), remanence-to-saturation ratio (M_r/M_s)and coercivity (H_c) are listed in Table 2. As shown in Fig. 6a, RGO–ZnNi₅-1, RGO–ZnNi₅-2 and RGO–ZnNi₅-3 show the similar M_s values. However, the M_s value of the bare ZnNi₅ particles (Fig. 6a) is slightly bigger than those of the RGO–ZnNi₅ nanocomposites, which could be attributed to the bigger particle size.³¹ Because of similar shape anisotropy, these samples show similar H_c values (70–80 Oe) except that RGO–ZnNi₅-3 has a relatively higher H_c value. The bigger H_c value may originate from its smaller size, which may change the magnetization reversal mechanism and lead to a higher coercivity.⁴⁴

Fig. 6 Magnetic hysteresis loops of the as-prepared samples measured at 300 K (the inset of (a) shows the magnetic separation of the composite).

Table 2 Magnetic data of the prepared samples

Samples	M s (emu g^−1 of ZnNi)	M r (emu g^−1 of ZnNi)	M r/Ms	H c (Oe)
ZnNi5	14.95	3.02	0.20	80.45
RGO–Ni	16.10	5.10	0.32	115.30
RGO–ZnNi5-1	12.70	2.35	0.19	78.23
RGO–ZnNi5-2	12.54	1.75	0.18	75.03
RGO–ZnNi5-3	12.67	2.31	0.18	90.45
RGO–ZnNi4	9.95	1.42	0.14	67.54
RGO–ZnNi3	7.84	0.69	0.09	60.11
RGO–ZnNi2	4.70	0.50	0.11	55.48

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The magnetic hysteresis loops of the composites with different Zn/Ni molar ratios are shown in Fig. 6b. The M_s value of Ni in RGO–Ni nanocomposite is calculated to be 16.10 emu g⁻¹ at T = 300 K, which is much lower than the reported value of bulk Ni (55 emu g⁻¹).⁴⁵ This may be attributed to the small particle size of Ni nanoparticles in the composites, which can decrease the effective magnetic moment of Ni.⁴⁶ With the increasing of Ni content in the RGO–ZnNi composites, the corresponding M_s and H_c values increase, which is in agreement with the reported FeNi alloys.⁴⁷ Meanwhile, there are no big change of the smaller M_r/M_s values at T = 300 K for all the samples (all smaller than 0.32), suggesting that the samples have similar soft magnetic characteristics.⁴⁸ Such RGO–ZnNi composites with soft-magnet behavior would have potential application in drug delivery, bioseparation, water treatment, and magnetic resonance imaging applications. In addition, due to the smaller size, the nanoparticle catalyst is generally hard to be recycled. However, our RGO–ZnNi nanocomposites with considerable M_s values make them easily be recollected from various dispersions by an external magnet (inset of Fig. 6a).

3.3. Catalytic properties of RGO–ZnNi nanocomposites

To quantitatively evaluate the catalytic activity of the as-synthesized nanocomposites, the reduction of 4-NP by NaBH₄ was employed as a model reaction, which could be easily monitored by time-dependent UV-vis absorption spectra (Fig. S2† and Fig. S3†). It is well known that 4-NP exhibits a strong absorption peak at 400 nm in alkaline solution.^49,50 As the reduction reaction proceeds, the intensity of the absorption peak at 400 nm gradually decreases. Meanwhile, new peaks at 300 nm from 4-AP appear with their intensity increasing with reaction time, indicating the gradual conversion of 4-NP to 4-AP.⁵¹ Among these RGO–ZnNi nanocomposites, RGO–ZnNi₅-2 shows the highest catalytic activity, with which the reaction finished within 8 min (Fig. S2a†), while the RGO–ZnNi₂ shows the lowest catalytic activity and cannot achieve the full reduction of 4-NP even with a reaction time of 50 min (Fig. S2d†). Moreover, compared with RGO–ZnNi₅, the bare ZnNi₅ shows much slower catalytic process with more than 50 min to finish the reaction (Fig. S3d†).

Pseudo first order kinetics is often applied for the evaluation of rate constants in this reaction.⁵² In the reaction system, the ratio of C_t to C₀ (C_t/C₀) (C_t and C₀ are the concentration of 4-NP at time t and 0, respectively) was measured from the ratio of the absorbances (A_t/A₀) at 400 nm. Good linear correlations of ln (A_t/A₀) versus time t are observed for various tested catalysts (Fig. S2f† and S3f†). The apparent rate constants (k_app), obtained from the slopes of the linearly fitted plots of ln (A_t/A₀) − t, are provided in Table 3. The sample of RGO–ZnNi₅-2 has the highest k_app value. The k_app values exhibit that the catalytic activities of RGO–ZnNi nanocomposites increase in the following order: RGO–ZnNi₂ < RGO–ZnNi₃ < RGO–ZnNi₄ < RGO–ZnNi₅, that is, the catalytic activities increase with the increasing Ni/Zn molar ratio in the ZnNi alloys. This is reasonable because metal Ni, as a commercial catalyst for the reduction of 4-NP, possesses much better catalytic activity than metal Zn.⁵³ In addition, it is well-known that the catalytic activity of a nanoparticle is strongly dependent on its size.^54,55 Usually, a decrease of the metal particle size leads to an increase in the fraction of low coordination metal sites such as vertices and edges on their surface, which can facilitate the reduction of 4-NP.⁵⁶ As mentioned above, the size of ZnNi alloys nanocrystals on RGO sheets decreases with the increasing Ni/Zn molar ratio, which also contributes to the increasing catalytic activity. However, RGO–Ni with higher Ni content and smaller particle size shows lower catalytic activity than RGO–ZnNi₅, which could be ascribed to the synergetic chemical coupling effects between Zn and Ni components in RGO–ZnNi₅.¹⁰

Table 3 Catalytic rate constants of the NaBH₄ reduction of 4-NP in the presence of the various nanocomposites and the correlation coefficient for the ln (C_t/C₀) − t plots

Samples	RGO–ZnNi5-1	RGO–ZnNi5-2	RGO–ZnNi5-3	RGO–ZnNi4	RGO–ZnNi3	RGO–ZnNi2	RGO–Ni	RGO	ZnNi5
k app (min^−1)	0.1768	0.2354	0.1887	0.0841	0.0647	0.0481	0.1202	0.0118	0.0475
R ^2	0.9981	0.9502	0.9848	0.9911	0.9971	0.9985	0.9970	0.9750	0.9993

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In comparison with bare ZnNi₅ (k_app = 0.0475 min⁻¹), the catalytic activities of RGO–ZnNi₅ are much higher (up to 5 times that of bare ZnNi₅). The rate constants obtained here are comparable to that of Au-based nanoparticles and GO/SnO₂/Pt.^57,58 As shown in Table 3, the RGO shows poor catalytic activity and the k_app value is only 0.0118 min⁻¹. We consider that the higher catalytic activities of RGO–ZnNi₅ than bare ZnNi₅ could be attributed to the following two sides. One side, RGO plays an important role in the enhanced catalytic activity. The RGO sheets with higher adsorption ability towards 4-NP via π–π stacking interactions would provide a drive for enriching 4-NP molecules to the catalyst, ZnNi₅ nanoparticles.^52,59 On the other hand, ZnNi₅ nanoparticles formed on the RGO sheets have much smaller sizes than bare ZnNi₅ particles, which also improves the catalytic activity of RGO–ZnNi₅.

It was found that a suitable loading amount of ZnNi₅ is also crucial for optimizing the catalytic activity of RGO–ZnNi₅ nanocomposites. As shown in Table 3, the catalytic activities of the samples with different loading amount of ZnNi₅ increase in the following order: RGO–ZnNi₅-1 < RGO–ZnNi₅-3 < RGO–ZnNi₅-2. The difference in catalytic activity can be attributed to the different loading amounts and particle sizes of ZnNi₅ on RGO. It can be seen that with the increase of ZnNi₅ loading amount, the catalytic activity increased at first and then decreased. That is reasonable that the catalytic activity increases with the increase of ZnNi₅ loading amount within a certain range because of the increase in the catalytically active sites. However, when the ZnNi₅ loading amount is further increased over its optimum value, the catalytic performance deteriorates, which may be due to the bigger particle size. As evidenced by TEM images in Fig. 5, RGO–ZnNi₅-1 with the highest ZnNi₅ loading amount shows much bigger particles size than RGO–ZnNi₅-2, which would result in lower catalytic activity of RGO–ZnNi₅-1 (k_app = 0.1768 min⁻¹) than RGO–ZnNi₅-2 (k_app = 0.2354 min⁻¹). Therefore, the composition (Ni/Zn molar ratio), the particle size of ZnNi alloys and the synergetic chemical coupling effects between the two metal components all exert an important influence on the resulting catalytic activities of the RGO–ZnNi nanocomposites.¹⁰

Catalysts based on noble metal nanoparticles often suffer from poisoning by the reaction product during the catalytic process.^60,61 Thus, the improvement of the catalytic stability is the key priorities for practical applications. In order to evaluate the catalytic stability of RGO–ZnNi nanocomposites, the representative samples of RGO–ZnNi₅-2 and RGO–Ni were tested for reusability in the reduction of 4-NP by NaBH₄ for five times and the k_app values for the successive five cycles is shown in Fig. 7. It can be seen that the catalytic efficiency of RGO–ZnNi₅-2 has no loss after five cycles, indicating the excellent stability of RGO–ZnNi₅-2 catalyst (Fig. 7a) against poisoning by the product of the reaction. In contrast, RGO–Ni (Fig. 7b) catalyst performs a slight decrease in the k_app values and lost its catalytic activity over the course of each round of reaction gradually. These results show that the introduction of Zn into Ni can effectively improve the catalytic stability of RGO–Ni. From the experimental results, it is concluded that the RGO–ZnNi nanocomposites are excellent catalysts with characters of high efficiency, low cost and magnetic separation for the reduction of 4-NP into 4-AP, which endow the possibility of their practical applications.^58,62,63

Fig. 7 The catalytic performance within five cycles for (a) RGO–ZnNi₅-2 and (b) RGO–Ni catalysts.

4. Conclusions

In summary, the RGO–ZnNi nanocomposites have been synthesized by in situ growth of ZnNi alloy nanocrystals accompanied by a co-reduction process. The size and morphology of ZnNi alloy on RGO sheets can be tuned by simply changing initial metal ion concentration and molar ratio of Zn²⁺ to Ni²⁺ in the EG solution. These RGO–ZnNi nanocomposites show ferromagnetic behavior and composition dependent magnetic properties. Especially, the RGO–ZnNi nanocomposites show excellent catalytic activity and extraordinary stability towards the reduction of 4-NP by NaBH₄ with apparent rate constant as high as 0.2354 min⁻¹, which is comparable to that of noble metal catalysts. Furthermore, the composites are much cheaper and could be easily magnetically separated for the reuse. It is expected that the RGO–ZnNi nanocomposites may find practical applications in catalysis and related areas.

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation of China (nos. 51272094, 51072071 and 51102117) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20123227110018).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S3. See DOI: 10.1039/c3ra45210g

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