A general strateg

z

loped to synthesize metal nanoparticles by solid-state redox reaction

s

e

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especially noble metal nanoparticles, such as Rh, Pd, Ag, PtCu ,and AuPt.1318 However, the promethod are complicated and timdicult to fabricate 3d-transitioparticles because of the oxidatiomedia.1921 Thermal decomposprecursor was developed for thnanoparticles including metallicNevertheless, its operation condiand toxic organic reactants arecould also be obtained by solvnumber of nanoparticles, such

conditions, easy handling, low cost, and easy scale-up.

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3. Key Laboratory of Material and Technology for Clean Energy, Ministry of Education,Key Laboratory of Advanced Functional Ma

Applied Chemistry, Xinjiang University, U

jdz@xju.edu.cn; Fax: +86-991-8580032; Tel

This paper is dedicated to Professor Xibirthday.

Electronic supplementary informationpatterns of the metal nanoparticles; BETDOI: 10.1039/c3ta14427e

This journal is The Royal Society of C3

cedures of the solution-basede-consuming. In addition, it isn metal (Fe, Co, and Ni) nano-n of the products in aqueousition of the organometallice synthesis of various metalFe and Co nanoparticles.2225

tions are harsh, and expensiveneeded. Metal nanoparticlesentless synthesis.26,27 A largeas metallic Pd, Ag, and Au

Furthermore, this general strategy may be extended for thesynthesis of other types of metal nanoparticles. We alsodemonstrated one application of the obtained Ni nanoparticleswhich were tested as catalysts for the reduction of 4-nitrophenol(4-NP) to 4-aminophenol (4-AP).

Experimental sectionStarting materials

All chemicals were used without further purication. Nick-el(II) chloride hexahydrate (NiCl2$6H2O, 99.0%), iron(III)nitrate nonahydrate [Fe(NO3)3$9H2O, 98.5%], tin(II) chloridenanoparticles byambient conditio

Yizhao Li, Yali Cao and Dian

A general strategy has been deve

under ambient conditions. A serie

bimetallic NiCo nanoparticles wer

(NaBH4). The obtained Ni nanopa

nitrophenol to 4-aminophenol. Th

nanoparticles.

Introduction

Metal nanoparticles with excellent optical, electrical, magnetic,and catalytic properties are attractive candidates for applica-tions in biomedicine, energy conversion, magnetic data storage,catalysis, and other elds.16 It is important to obtain metalnanoparticles by a facile approach, not only for fundamentalscientic research, but also for various technological applica-tions.712 Thus, a large number of researchers have dedicatedeorts to the preparation of metal nanoparticles.

In the past few decades, a variety of methods for thesynthesis of metal nanomaterials have been proposed. Solution-based reduction routes have been used for the preparation ofvarious metal nanomaterials with dierent morphologies,

Cite this: J. Mater. Chem. A, 2014, 2,3761

Received 30th October 2013Accepted 2nd January 2014

DOI: 10.1039/c3ta14427e

www.rsc.org/MaterialsAterials, Autonomous Region, Institute of

rumqi, Xinjiang 830046, China. E-mail:

: +86-991-8583083

nquan Xin on the occasion of his 80th

(ESI) available: XRD, EDS, and XPSand BJH results of the catalysts. See

hemistry 2014of metallic nanoparticles, such as Ni, Fe, Sn, Co nanoparticles, and

prepared by using inexpensive metal salts and sodium borohydride

ticles showed excellent catalytic activity toward the reduction of 4-

protocol creates a novel and facile route for the synthesis of metal

nanoparticles, were synthesized by heating the precursor metalsalts in the absence of solvents.2830 But a high temperature isgenerally required in the route. The development of thesemethods introduces the possibility of synthesizing metalnanoparticles without solvent. However, it is still a challenge formaterials scientists to develop a general facile process by usingsimple materials to synthesize metal nanoparticles.

Here, we report on a general strategy for the synthesis ofvarious metal nanoparticles by a solid-state redox route underambient conditions. Metallic Ni, Fe, Sn, Co nanoparticles andbimetallic NiCo nanoparticles have been prepared through theone-pot redox route using the corresponding inorganic saltsand sodium borohydride (NaBH4) in the solid-state form. Thesynthesis approach has several advantages, including mildy for synthesis of metala solid-state redox route underns

eng Jia*

View Article OnlineView Journal | View Issuedihydrate (SnCl2$2H2O, 98.0%), cobalt(II) acetate tetrahy-drate [Co(CH3COO)2$4H2O, 99.0%], sodium borohydride(NaBH4, 98%), and 4-nitrophenol (4-NP, 99%) were obtainedfrom Tianjin Fuchen Chemical Reagents Co., Ltd. Sodiumchloride (NaCl), sodium dodecyl sulfate (SDS), and poly-ethylene glycol 400 (PEG-400) were purchased from TianjinGuangfu Chemical Reagent Co., Ltd.

J. Mater. Chem. A, 2014, 2, 37613765 | 3761

NaBH4 (0.15 g, 4 mmol) were mixed and ground in an agate

Co(CH3COO)2$4H2O (0.50 g, 2 mmol). The yield was 92.5%.

20.0 mL of 4-NP solution (0.1 mM) in a conical ask. Then,2.0 mg of the as-prepared Ni nanoparticles was added to themixture as a catalyst. Parts of the mixture were withdrawn every2 min and the concentrations of 4-NP were determined byUV-vis absorption spectroscopy (Hitachi U-3900H UV-vis spec-trophotometer) at a wavelength of 400 nm. In the recyclingstudy, the catalyst was separated from the solution bycentrifugation and rinsed with water. Then, it was reused in thenext reaction run.

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View Article OnlineFor NiCo nanoparticles: the same procedure was applied asfor Ni nanoparticles except that NiCl2$6H2O (0.24 g, 1 mmol)and Co(CH3COO)2$4H2O (0.25 g, 1 mmol) were used simulta-neously. The yield was 90.8%.

Characterization

The crystal structure of the obtained samples was characterizedby X-ray diraction (XRD) using a DX-1000 X-ray diractometerwith Cu-Ka radiation (l 1.5418 A). X-ray photoelectron spec-troscopy (XPS) signals were collected on a Thermo ESCALAB 250instrument using a monochromated Al X-ray resource at 1486.6eV operated at 15 kW. Field emission scanning electronmicroscope (FESEM) images were obtained on a HitachiU-3900H scanning electron microscope with an acceleratingvoltage of 20 kV. Transmission electron microscope (TEM) andhigh resolution transmission electron microscope (HRTEM)images of the products were obtained on a JEOL JEM-2010Felectron microscope with an accelerating voltage of 200 kV. Theproduct component measurement was carried out by energydispersive X-ray spectroscopy (EDS) using a JEOL JEM-2010Felectron microscope. The BrunauerEmmettTeller (BET) andBarrettJoynerHalender (BJH) results were obtained on aMicromeritics ASAP 2020 surface area and porosity analyzer.

Catalytic reduction of 4-NP

The reduction of 4-NP was studied as a model reaction toconrm the catalytic activity of the synthesized Ni nano-particles. The following procedure was used for the catalytictest: 10.0 mL of fresh NaBH4 solution (0.1 M) was mixed withmortar at room temperature. Subsequently, several drops ofwater were slowly added into the mixtures. Accompanied by therelease of heat and vapor, the color of mixtures changed fromgreen to black. Aer grinding for about 30 min, the resultingsolid products were washed with distilled water and absoluteethanol several times. The products were then dried at 60 C for2 h in a vacuum drying oven. The yield was 95.2%. In addition,NaCl (2.0 g) and SDS (5.77 g) were added into the mixtures tofabricate the Ni nanoparticles prior to the addition of water,respectively.

For Fe nanoparticles: the same procedure was applied as forNi nanoparticles except that NiCl2$6H2O was replaced byFe(NO3)3$9H2O (0.81 g, 2 mmol). The yield was 92.7%.

For Sn nanoparticles: the same procedure was applied as forNi nanoparticles except that NiCl2$6H2O was replaced bySnCl2$2H2O (0.45 g, 2 mmol), and PEG-400 (1.0 mL) was added.The yield was 91.3%.

For Co nanoparticles: the same procedure was applied as forNi nanoparticles except that NiCl2$6H2O was replaced byPreparation of metal nanoparticles

All metal nanoparticles in this work were prepared by the solid-state redox method under ambient conditions. Typical exam-ples are given below.

For Ni nanoparticles: NiCl2$6H2O (0.48 g, 2 mmol) and3762 | J. Mater. Chem. A, 2014, 2, 37613765Scheme 1 The synthesis of metal nanoparticles by a solid-state redoxroute under ambient conditions.Results and discussionThe formation of metal nanoparticles

The synthesis was carried out by a facile route of solid-stateredox reaction under ambient conditions. The overall syntheticprocedure for metal nanoparticles is depicted in Scheme 1.Firstly, metal salts were mixed with NaBH4 in an agate mortar atroom temperature. The starting materials were ground toensure full mixing of the reactants. Additives such as NaCl, SDS,and PEG-400 can be introduced into the system as cappingagents and/or dispersing agents to inuence the morphologyand size of the products. Then, several drops of water wereslowly added into the mixtures to trigger the reaction. Accom-panied by the release of heat and vapor, the color of themixtures changed, which suggests that solid-state reactionoccurred. Water was used as a reactant in the reaction. Theoverall solid-state redox reaction can be described by thefollowing equation:

2Mx+ + 2xBH4 + 6xH2O/ 2M

0 + 2xB(OH)3 + 7xH2 (1)

where Mx+ represents the metal ions and M0 is the corre-sponding metal. The metal salts of the reactants can be reducedby NaBH4 to produce the metals in the presence of a smallamount of water. Based on the solid-state reactions,31 the metalparticles with nanometer size can be obtained.

This solid-state synthesis of metal nanoparticles demon-strates several advantages: (1) toxic and expensive organicsolvents are not used in the reaction and the starting materialsare inexpensive and easily obtainable; (2) the synthesis is con-ducted at room temperature under atmospheric pressure,which is facile, cost-eective, and eco-friendly; (3) the compo-sition, size, and morphology of the nanoparticles can be tunedby the types of metal salts and additives; (4) the synthesis can beeasily up-scaled; and (5) the protocol may be further extended tosynthesize other metal nanoparticles such as noble metals andbimetals.This journal is The Royal Society of Chemistry 2014

Structure and morphology of the products

Metallic Ni has excellent magnetic and catalytic properties,which are widely used as magnetic storage devices and catalyticmaterials.32,33 Herein, Ni nanoparticles have been prepared bythe solid-state chemical reaction approach. The XRD pattern ofthe obtained sample is shown in Fig. 1a. All the reections areindexed to cubic phase Ni and are in agreement with JCPDS04-0850. XPS was employed to investigate the compositions ofthe product in detail. Fig. 1b shows the photoelectron spectrumfor the Ni 2p region of the Ni nanoparticles. The peaks at 852.3and 855.8 eV appear, which indicates the presence of themetallic and oxidative Ni in the nanoparticles.34,35 The oxidizedNi may come from partial oxidation during the preparation andwashing processes. Fig. 1c and d show the FESEM and TEMimages of the Ni nanoparticles. Irregular nanoparticles with thesize of 100500 nm were obtained when nickel chloride wasused as a starting material without any additives. The inset ofFig. 1d clearly shows that the particle surface is enclosed with alayer of oxides.

The morphology of nanoparticles can be inuenced by theadditives. When NaCl was added, uniform Ni particles with asize of about 20 nm were obtained (Fig. 2a and b). As shown inFig. 2c and d, nanoparticles of 2050 nm were prepared aeradding SDS. In addition, the XRD patterns (Fig. S1) show that

chloride as a reactant led to the formation of tetragonal Sn

Fig. 2 FESEM and TEM images of Ni nanoparticles with dierentadditives: (a and b) Ni nanoparticles with NaCl; (c and d) Ni nano-particles with SDS and the insets are the corresponding HRTEMimages.

Fig. 3 XRD patterns of metal nanoparticles (a: Fe; d: Sn; g: Co); FESEMimages of metal nanoparticles (b: Fe; e: Sn; h: Co); TEM images ofmetal nanoparticles (c: Fe; f: Sn; i: Co).

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View Article Onlinethe crystal structures of the samples are not changed with theaddition of additives. The EDS analyses of the three kinds of Ninanoparticles were conducted (Fig. S2). They illustrate that theas-prepared structures consist of Ni and O.

Metallic Fe, Sn, Co nanoparticles and bimetallic NiConanoparticles have potential applications in magnetic datastorage, catalysis, and energy storage.3639 They can also befabricated by the general route. Fig. 3 shows XRD patterns,

Fig. 1 (a) XRD pattern of Ni nanoparticles; (b) XPS spectrum of Ni 2p ofNi nanoparticles; (c) FESEM image of Ni nanoparticles; (d) TEM imageof Ni nanoparticles and the inset is a HRTEM image of thenanoparticles.This journal is The Royal Society of Chemistry 2014FESEM images, and TEM images of the obtained Fe, Sn, and Conanoparticles with dierent metal salts. The characteristicpeaks of the XRD pattern in Fig. 3a can be indexed as the cubicFe (JCPDS 06-0696). From Fig. 3b and c, it can be seen thatspherical Fe nanoparticles with a diameter of about 10 nm wereobtained when iron nitrate was used as a starting material. TinJ. Mater. Chem. A, 2014, 2, 37613765 | 3763

(JCPDS 04-0673) (Fig. 3d). The Sn nanoparticles with the size of20100 nm are shown in Fig. 3e and f. When cobalt acetate wasused as a starting material, cubic Co (JCPDS 15-0806) wasobtained (Fig. 3g). The Co nanoparticles (Fig. 3h and i) have anirregular shape with a size of 2050 nm. In addition, the XPSspectra of the Fe, Sn, and Co nanoparticles are presented inFig. S3. It shows that the metal nanoparticles are in zero valentand positive valent states.4042 This conrms that metals andoxides are included in the resulted samples.

When two kinds of metal salts were added in the reactionsystem, bimetallic nanomaterials can be prepared by ourproposed protocol. NiCo nanoparticles have been synthesizedwith nickel chloride and cobalt acetate as starting materials.The XRD pattern of the sample (Fig. 4a) is very similar to that ofeither Ni (JCPDS 04-0850) or Co (JCPDS 15-0806). The positionsof peaks lie that of pure Ni or Co. XPS is applied to furtherconrm the formation of NiCo nanoparticles. The Ni 2p peaksat 849.2 and 854.6 eV are assigned to Ni(0) and Ni(II), respec-tively (Fig. 4b). For Co 2p, three peaks at 779.8, 795.2, and 782.2

Catalytic properties of Ni nanoparticles

Metal nanoparticles have potential applications in manyareas. We chose Ni-catalyzed reduction of 4-NP as a modelreaction to demonstrate the use of our metal nanoparticles asecient catalysts. The catalytic properties of the as-preparedNi nanoparticles with dierent additives were investigated.For simplicity, Ni nanoparticles obtained without additives,and with NaCl and SDS are designated as Ni-1, Ni-2 and Ni-3,respectively. Fig. 5ac show the UV-vis spectra as a functionof reaction time for the reduction of 4-NP in the presence ofthree kinds of catalysts. Clearly, with the Ni nanoparticles,the characteristic absorption peak of 4-NP at 400 nm gradu-ally decreased, while a new peak at 295 nm, ascribed to 4-AP,appeared. Fig. 5d and e show C/C0 and ln(C/C0) versusreaction time for the reduction of 4-NP over Ni-1, Ni-2, andNi-3. It can be observed that Ni-2 and Ni-3 exhibited goodperformance compared with Ni-1. From Fig. 5d, we canobserve that the induction time (t0) was 8, 0, and 2 minutesfor Ni-1, Ni-2 and Ni-3, respectively. As shown in Fig. 5e, the

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View Article OnlineeV indicate that Co(0) and Co(II) are dominant (Fig. 4c). Thesignals of metallic bonding energy in the as-obtained NiConanoparticles are slightly shied compared with pure Ni and Cometals due to the dierent environment of Ni and Co in theNiCo structure and pure Ni and pure Co metallic materials.43,44

The EDS pattern of NiCo nanoparticles (Fig. 4d) reveals that theatomic ratio of Ni to Co is 54.8 : 45.2. From Fig. 4e and f, it canbe seen that a number of irregular nanoparticles with the size of30100 nm were obtained.

The stability of the metal nanoparticles has also beeninvestigated. The samples were stored in air for two weeks. Thenthe crystal structure of the metal nanoparticles was character-ized by XRD. As shown in Fig. S4, the XRD patterns of thesamples exposed to air for two weeks remain almost unaltered,compared with those of fresh metal nanoparticles. It indicatesthat the metal nanoparticles prepared by a solid-state redoxroute have good stability. The layer of oxides on the surface ofthe particles oers protection to the metal nanoparticles fromfurther oxidation.23

Fig. 4 (a) XRD pattern of NiCo nanoparticles; (b) XPS spectra of Ni 2pof NiCo nanoparticles; (c) XPS spectra of Co 2p of NiCo nanoparticles;(d) EDS patterns of NiCo nanoparticles; (e) FESEM image of NiConanoparticles; (f) TEM image of NiCo nanoparticles.3764 | J. Mater. Chem. A, 2014, 2, 37613765rate constants (k) of Ni-1, Ni-2, and Ni-3 were calculated to be0.176, 0.291 and 0.333 min1, respectively. Based on the rateconstants, Ni-3 showed a higher catalytic activity than Ni-2.The nitrogen adsorptiondesorption isotherm measure-ments were carried out to examine the specic surface areasof the catalysts. As shown in Fig. S5, Ni-1, Ni-2, andNi-3 manifest specic surface areas of 25.6, 38.2, and44.0 m2 g1, respectively. The excellent catalytic performanceof Ni-3 may be ascribed to its large specic surface area,which renders to more active sites for the catalytic process.45

The catalytic activity of the Ni nanoparticles is comparable tothose reported in the literature.4649 The reusability isvery important in the catalytic reaction for practical appli-cations. In the study, Ni-3 was employed as a representativecatalyst in the reduction of 4-NP over ve cycles. From Fig. 5f,it is found that the conversion of 4-NP could still reach 86.6%aer ve cycles, indicating that Ni-3 exhibited goodrecyclability.

Fig. 5 Catalytic performance of the obtained Ni nanoparticles: UV-vis spectra during the catalytic reduction of 4-NP in the presence ofNi-1 (a), Ni-2 (b), and Ni-3 (c); plot of C/C0 versus reaction time (d)and plot of ln(C/C0) versus reaction time (e) for the reduction of 4-NPin the presence of Ni nanoparticles; (f) conversion of 4-NP in vecycles with Ni-3.This journal is The Royal Society of Chemistry 2014

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View Article OnlineConclusions

In summary, we have demonstrated a general strategy toprepare metal nanoparticles by solid-state redox reaction. Themetallic Ni, Fe, Sn, Co and bimetallic NiCo nanoparticles havebeen successfully synthesized by using inexpensive metal saltsand NaBH4. The route is facile, scalable, and cost-eective. Itmay be extended to the synthesis of other metal nanoparticles.The obtained metal nanoparticles with excellent properties areexpected to have a wide range of applications, such as catalysts,electronic devices, and battery materials.

Acknowledgements

This work was nancially supported by the Doctoral InnovationProgram of Xinjiang University (no. XJUBSCX-2012019), theGraduate Research Innovation Project of Xinjiang (no.XJGRI2013020), the National Natural Science Foundation ofChina (no. 21101132, 21361024 and 21271151) and the Programfor Changjiang Scholars and Innovative Research Team in theUniversity of Ministry of Education of China (IRT1081).

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A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...

A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...

A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...A general strategy for synthesis of metal nanoparticles by a solid-state redox route under ambient conditionsThis paper is dedicated to Professor...