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Materials Science and Engineering B 176 (2011) 1021– 1027

Contents lists available at ScienceDirect

Materials Science and Engineering B

jou rna l h om epage: www.elsev ier .com/ locate /mseb

hort communication

auliflower-like CuI nanostructures: Green synthesis and applications as catalystnd adsorbent

i Jiang, Shuyan Gao ∗, Zhengdao Li, Xiaoxia Jia, Yanli Chenollege of Chemistry and Environmental Science, Henan Normal University, Xinxiang, Henan 453007, People’s Republic of China

r t i c l e i n f o

rticle history:eceived 30 December 2010eceived in revised form 28 March 2011ccepted 2 May 2011

eywords:

a b s t r a c t

Cauliflower-like CuI nanostructures is realized by an ampicillin-assisted clean, nontoxic, environmentallyfriendly synthesis strategy at room temperature. The morphology, composition, and phase structure of as-prepared powders were characterized by field emission scanning electron microscopy, X-ray diffraction,and X-ray photoelectron spectroscopy. The results show that ampicillin plays dual roles, reducing andmorphology-directing agents, in the formation of the products. A possible growth mechanism of the

reen chemistryuIatalystdsorbent

cauliflower-like CuI nanostructures is tentatively proposed. The preliminary investigations show that thecauliflower-like CuI structure not only exhibits high catalytic activity with respect to coupling reactionbetween benzylamine and iodobenzene but also possesses high removal capacity for Cd (II), which maybe ascribed to the high specific surface area of the special configuration. To the best of our knowledge, it isthe first report that cauliflower-like CuI nanoparticles act as catalyst for coupling reaction and adsorbent

for heavy metal ion.

. Introduction

Cuprous iodide (CuI) has attracted much attention in theast few decades owing to its unusual characters such as largeirect band gap, negative spin-orbit splitting, unusually large tem-erature dependency, anomalous diamagnetism behavior, large

onicity, new high pressure phase etc. [1–4] and potential appli-ations in superionic conductor, solid-state solar cells, catalysis forynthesis of organic compounds, etc. [5–11]. In this context, a greatffort has been made to develop different methods to prepare CuIanocrystals [5], porous CuI nanoparticles [7], and nearly sphericalnd flower-like CuI [12,13]. Even though such methods can success-ully produce highly crystalline and uniformly sized nanoparticles,hese synthetic procedures are not exempt of drawbacks, becausehey required toxic raw materials, complicated synthetic steps, origh reaction temperature. Especially, the methods reported to dateften use reducing agents, such as sodium sulfite and sodium boro-ydride, which are highly reactive chemicals and pose potentialnvironmental and biological risks and thus limit their exploita-ion at the application level. Therefore, it is required to develop alean, facile, and friendly method to synthesize CuI nanostructuresn large-quantity under mild conditions.

Along with the progressive emphasis on the topic of “green”hemistry, utilization of nontoxic chemicals, environmentallyenign solvents and renewable materials are some of key issues

∗ Corresponding author.E-mail address: shuyangao@htu.cn (S. Gao).

921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2011.05.023

© 2011 Elsevier B.V. All rights reserved.

in the nanomaterial science field [14–22]. As we know, ampi-cillin is a mild, inexpensive, and nontoxic reducing agent [23]. Itsbenign water-solubility makes fabrication of nanocrystals process-ing in water medium, which is very crucial for “green chemistry”.Moreover, the special configuration and large amount of functionalgroups in ampicillin endow it with unique structure-directingfunction in nanomaterials synthesis. Herein, we describe a facilegreen synthetic route to construct cauliflower-like CuI nanos-tructures (abbr. CuI cauliflower) using ampicillin as reducing andmorphology-stabilizing agent. This route is capable of synthesizinghigh-purity cauliflower-like CuI nanostructures at room temper-ature in short time. In comparison with the reported protocols,our method is a green, environment-friendly, time-saving, anddirect one-step process. The as-formed cauliflower-like CuI pos-sesses high specific surface area, which endows it with excellentcatalytic activity and high adsorption capacity. As a demonstration,the synthesized CuI cauliflower is tested for catalyzing couplingreaction between benzylamine and iodobenzene and removing Cd(II) from water, which shows excellent catalytic performance andhigh removal capacity. To the best of our knowledge, it is the firstreport that CuI cauliflower acts as catalyst for coupling reaction andadsorbent for heavy metal ions.

2. Experimental

2.1. Preparation of CuI cauliflower

All chemical reagents were of analytical grade and used withoutfurther purification. All water used in this investigation was deion-

1 nd Engineering B 176 (2011) 1021– 1027

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zed by a nanopure filtration system to a resistivity of 18 M� cm.he preparation of ampicillin-assisted CuI cauliflower is quitetraightforward. 5 mL of 0.15 M CuSO4 aqueous solution, 7 mL of.005 M ampicillin aqueous solution and 5 mL of 0.15 M KI aque-us solution were added under constant stirring, respectively, to3 mL of ultrapure water. The mixture was stirred for 3.5 h at roomemperature. When the reaction was completed, gray precipitatend colorless supernatant were obtained. The resulting precipitateas filtered and washed with distilled water and finally dried in airaturally.

.2. Sample characterizations

The product was characterized by field emission scanninglectron microscopy (FESEM), X-ray diffraction (XRD), and X-rayhotoelectron spectroscopy (XPS) techniques to obtain detailed

nformation on the morphology, component, and crystallinetructure. The FESEM images were obtained on an XL30 ESEMEG scanning electron microscopy operating at 20 kV. The XRDattern was recorded in the 2� range of 20–80◦ on a Rigaku-/Max 2500 V/PC X-ray diffractometer using Cu k�1 radiation

� = 1.54056 A) at 40 kV and 200 mA. XPS was collected on anSCALab MKII X-ray photoelectron spectrometer, using non-onochromatized Mg K� X-ray as excitation source. The attached

nergy-dispersive X-ray analysis (EDX) probe allowed microsopiclemental analysis of the sample under FESEM.

.3. Catalytic tests

The coupling reaction between benzylamine and iodobenzene,sing the as-prepared CuI as catalyst, ethylene glycol as lig-nd and unpurified 2-propanol as the solvent, was performedithout protection from air or moisture. CuI (10 mg, 0.05 mmol)

nd potassium phosphate (425 mg, 2.00 mmol) were put into aeflon septum screw-capped test tube followed by the addition of-propanol (1.0 mL), ethylene glycol (111 �L, 2.00 mmol), benzy-

amine (131 �L, 1.20 mmol), and iodobenzene (112 �L, 1.00 mmol)y microsyringe at room temperature. The tube was capped andhe reaction mixture was heated at 80 ◦C and then allowed to coolo room temperature, followed by adding diethyl ether (2 mL) andater (2 mL). The isolated compound was analyzed by GC [24]. The

olid remainder entered next reaction to test the recycle of catalyst.

.4. Metal ion adsorption test

The experiments include adsorption and desorption studies.atch studies were employed in all of the adsorption runs. A solu-ion by diluting Cd (II) standard solution (1000 mg/L) in deionizedater was used as the Cd (II) source. In order to inspect adsorption

f Cd (II) on the container surface, control experiment was car-ied out without CuI. It was found that no adsorption occurred onhe container wall. In adsorption experiment, an amount of the as-repared CuI (0.015 g) was placed in a 50 mL Erlenmeyer flask, intohich 10 mL of each of Cd (II) solutions was added. The mixture was

gitated for 3 h and then placed for 21 h to establish equilibrium atoom temperature. After the equilibrium, the CuI powder was theneparated from the mixture by centrifugation. Cd (II) concentrationefore and after treatment was measured using atomic absorptionpectrophotometer (AAS). The isotherm studies were carried out atifferent initial Cd (II) concentrations between 100 and 900 mg/L.he amount of Cd (II) adsorbed per unit mass of CuI (Qe) was cal-

ulated using the following Eq. (1) based on Langmuir simulation25]:

e = (Ci − Ce)VM

(1)

Fig. 1. Typical XRD pattern of the sample. The vertical lines represent the data ofJCPDS card no. 06-0246.

Here, Ci is initial concentration of metal ion in solution (mg/L), Ce

is equilibrium concentration of cadmium ion (mmol/L or mg/L),V is volume, M is mass of the adsorbent (g), and Qe is amountadsorbed per unit mass of adsorbent (mg/g or mmol/g), respectively[25]. Adsorption/desorption cycles were performed to examinethe reusability and metal recovery efficiency of the adsorbent CuI.Each cycle consisted of loading with an aqueous Cd (II) solution(Co = 100 mg/L) and elution of the bound Cd (II) with 10 mL of0.001 M Y solution for 3 h. The desorbed Cd (II) was collected andestimated using AAS. The desorption ratio (Dr) was calculated usingthe following Eq. (2):

Dr = amount of metal desorbed to the elution mediumamount of metal absorbed on the adsorbent

× 100%

(2)

The regenerated adsorbent CuI was again tested for furtheradsorption of Cd (II). Adsorption and desorption experiments werefollowed for 3 cycles.

3. Results and discussion

3.1. Structural analysis of cui cauliflower

Powder X-ray diffraction (XRD) pattern of the as-prepared sam-ple (Fig. 1) reveals the formation of a single cubic phase CuI witha Marshite, syn structure (JCPDS 06-0246). No other diffractionpeaks arising from metallic Cu or Cu oxides appear in the XRD pat-tern indicating the high phase purity of the as-prepared sample.To obtain chemical states of the elements within the sample, weperformed detailed analysis of X-ray photoelectron spectroscopy(XPS) spectra. The binding energies were corrected for specimencharging by calibrating the C1s peak to 284.6 eV. Fig. 2A shows thatthe positions of the peaks of Cu 2p1/2 and 2p3/2 of the sample CuIare 952.1 and 932.1 eV with very weak shakeup, which implies afeature of Cu+ [26]. The XPS spectrum of Cu 2p region was decon-vulated to identify % distribution of Cu (I) and Cu (II) as 95% and5%, respectively. As shown in Fig. 2B, the peak positions of I 3d5,619.0 and 630.4 eV, are well consistent with the ones in database.

In order to further confirm the elemental ratio, EDX spectrum wascollected, as shown in Fig. 3, which displays peaks originating fromCu, I, and C only. An elemental ratio of Cu:I of unity is in good agree-ment with the stoichiometric ratio. The C signal (at 0.25 eV) of the

Y. Jiang et al. / Materials Science and Eng

635630625620615

B

I 3d5

Inte

nsity

(a.u

.)

Bind ing En ergy (eV )

960955950945940935930925

Inte

nsity

(a.u

.)

Bind ing Ene rgy (eV)

Cu 2p 3/2

Cu 2p 1/2

A

Fig. 2. High-resolution XPS spectra taken for the Cu and I region of the sample: (A)Cu 2p and (B) I 3d5.

109876543210

0

200

400

600

800

1000

csp/

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keV

C

I

Cu

I

Cu

Fig. 3. EDX spectrum of the CuI sample prepared with ampicilion.

ineering B 176 (2011) 1021– 1027 1023

EDX plots belongs to the carbon grid used for nanoparticle depo-sition. Therefore, the XPS result together with EDS proves that thesample is composed of CuI.

In order to obtain detailed information about the microstructureand morphology of the as-synthesized sample, FESEM observa-tions are carried out and shown in Fig. 4. The low-magnificationimage (Fig. 4A) indicates that the panoramic morphology of theas-prepared sample is mainly composed of uniform, spheric archi-tectures ranging from 1.5 to 2.5 �m in diameter.

3.2. Growth mechanism

What growth mechanism is operative here for the formationof such cauliflower-like nanostructures? We proposed a forma-tion mechanism for CuI cauliflower in the presence of ampicillin(Scheme 1). It is believed that the growth of cauliflower-like struc-ture should be completed by two steps, i.e. nucleation and growth,respectively. Ampicillin is dispersed within liquid medium mainlyas a random coil sol bearing abundant negatively charged car-boxylic groups. When Cu2+ ions enter into aqueous solution, thereis electrostatic attraction between the positively charged Cu2+-water complex and the negatively charged carboxylic groups ofampicillin, thus forming complex Cu2+-ampicillin. Upon the intro-duction of I−, the redox reaction between Cu2+-ampicillin and I−

results in the formation of CuI crystal seeds (Scheme 1B). Thegradual attachment of reactive constituents onto the nucleationcenters directed by ampicillin molecules leads to the formationof nanoparticles (Scheme 1C). Although the exact mechanism forthe formation of these architectures has not been fully understood,the speculated reasons are discussed from the following aspect.The appearance of the cauliflower-like morphology might havebeen influenced by the increased stability inherent to the mor-phology. From Gibbs–Curie–Wulff theorem, the shape of a crystalis determined by the relative surface free energy of individual crys-tallographic faces. The final crystal shape results from minimizingthe total free energy of the system [27]. The surface energy ofthe spherical shape is the lowest [28], which has already madeCuO nanosheets self-assembly into spherical micropatterns [29],ZnO nanoclusters arrays self-assemble into spherical micropatterns[30], and copper nanoparticles self-organize in aqueous medium[31]. Such self-assembly represents a new type of coalescence pro-cess and may be the reason for the formation of the cauliflower-likenanoarchitecture in this case (Scheme 1D). More studies and workare underway to further investigate the mechanisms for the assem-bly process.

As is well known, a redox reaction occurs immediately whenCu2+ and I− are mixed in water solution at room temperature toform irregular CuI microparticles and brown iodic solutions. How-ever, it is colorless solutions that occur in the presence of ampicillin,which proves that ampicillin itself is responsible for this reductionreaction. Besides, the failure of producing CuI cauliflower whensubstituting ascorbic acid and L-tryptophan for ampicillin also fur-ther confirms the special role of ampicillin in the formation of suchunique cauliflower-like CuI (Figs. 4C–F). Those observations suggestthat the ampicillin serves not only as a reducing agent but also as astrong shape controller to assist the formation of CuI cauliflower.

3.3. Catalytic reaction

In recent years, the use of CuI as a catalyst for coupling reac-tion have been intensively studied because of their high theoreticalcapacity, environmental benignity, low cost, etc. [26,32,33]. The

remaining challenge is how to improve its efficiency. Althoughnanoparticles have been employed as heterogeneous catalysts forvarious organic transformations because of their large specific sur-face area [34,35], the investigation of CuI nanoparticles as catalysts

1024 Y. Jiang et al. / Materials Science and Engineering B 176 (2011) 1021– 1027

F scobici

ftctbscmadi

TT

ig. 4. (A, C and E) Typical FESEM images of the products prepared with ampicillin, amages.

or coupling reaction has seldomly been reported until now. So,o demonstrate the potential application of the as-prepared CuIauliflower as catalyst, we carried out a preliminary investiga-ion into their catalytic activity for the C-N bond-forming processetween benzylamine and iodobenzene (Scheme 2). The result washown in Table 1, from which it can be clearly seen that the CuIauliflower showed much better catalytic activity than the com-ercial powder just slightly lower than that of tetrahedral CuI [36],

nd the catalyst can be recycled at least three times without evi-ent loss of the catalytic activity. The catalysis process is shown

n Scheme 3 and outlined as follows: first, cuprous ion reacts with

able 1est of catalytic activity of CuI cauliflower for coupling reaction.

Entry Catalyst Reaction time (h) Yield [%]a

1 CuI cauliflower 9 95.192 Recovered from entry 1 9 93.453 Recovered from entry 2 9 92.324 Commercial CuI 9 69.02

a Yield of N-(phenyl)benzylamine.

acid and l-tryptophan respectively. (B, D and F) Corresponding high-magnification

benzylamine to form the chelate 3A, which coordinates with a suit-able iodobenzene to provide the complex 3B. Next, intramolecularnucleophilic substitution occurs at the aromatic ring to give thetransition state 3C. This step might be the rate-determining step,and the intramolecular attack would lower its activation energy.Finally, HI was removed from 3C with the assistance of potassiumphosphate to deliver another complex 3D, which could decomposeto produce the coupling product and regenerate the cuprous ion[33].

In our case, the improvement in the catalytic activity of the as-prepared CuI cauliflower may be attributed to the small size andhigh surface area. The BET surface areas of cauliflower-like CuI is2.0760 m2 g−1, higher than that of commercial CuI (whose mor-phology was shown in Fig. 5), 1.3436 m2 g−1. On the one hand, it isexpected to increase the nanoparticle surface tension by decreas-ing its size, which makes surface atoms very active. On the otherhand, due to their higher surface area, nanometer-sized materi-

als can exhibit very different catalytic property from those of thebulk counterparts [37]. The external surface of particles is used asa contact surface in catalytic reaction. The larger the external sur-face area of a catalyst, the higher the chance of reagent contacting

Y. Jiang et al. / Materials Science and Engineering B 176 (2011) 1021– 1027 1025

S -obtais ; (D) t

pIcaueksiir

3

va

Sb

cheme 1. Schematic illustration of the proposed formation mechanism for the astructure; (B) the formation of CuI crystal seeds; (C) the formation of nanoparticles

articles. The hierarchically micro/nano-structured morphology.t is well documented that nanomaterials, such as nanoparti-les and nanorods with high surface-to-volume ratios, tend toggregate during the preparation and catalysis process, whichndoubtedly results in the reduction of the catalytic efficiency. Anffective way to inhibit the nanoparticles from aggregation andeep the high catalytic efficiency is to organize these nanometer-caled materials into hierarchical structures [38]. This fundamentalnvestigation places solid foundation for the feasible and promis-ng application of such CuI cauliflower in catalyzing couplingeaction.

.4. Cadmium removal

It has been extensively and intensively realized that Cd (II) isery toxic, to which prolonged exposure causes kidney failure,nemia, cardiovascular diseases, growth impairment, and loss of

+ H2NBn

5 mol%CuI2 equ iv. K3PO42 equ iv. HO(CH2)2OH

isopropanol , 80oCN(H)BnI

cheme 2. The coupling reaction between benzylamine and iodobenzene catalyzedy the as-prepared CuI cauliflower.

ned CuI cauliflower. (A) Simple representation of the formation of cauliflower-likehe formation of nanoparticle-based cauliflower-like CuI.

taste and smell [39]. Therefore, Cd (II) has ranked high amongthe list of the carcinogens [40]. In order to remove toxic Cd (II)from wastewater, current methods are chemical precipitation,ion exchange, solvent extraction, adsorption, and reverse osmo-sis techniques [41]. Among the remedial technologies available,adsorption is popular because of its low cost and simplicity [42–44].Recently, nanometer-sized adsorbents have been used for wastew-ater treatment and shown remarkable potential because of theirhigh removal capacity originating from the large surface areas [45].The as-formed CuI cauliflower possesses high specific surface area,which is expected to enhance accessibility of adsorbates to the reac-tive sites. In order to demonstrate the potential application of theas-prepared CuI cauliflower as adsorbent for heavy metal ions, wecarried out a preliminary investigation into its removal capacity forCd (II). Fig. 6A shows a typical adsorption isotherm of Cd (II) on theas-prepared CuI cauliflower at 25 ◦C measured with different initialCd (II) concentrations. The Cd (II) removal capacity is analyzed to be153.3 mg/g. Compared with the Cd (II) removal capacities for ami-doximated polyacrylonitrile/organobentonite composite [46] andporous poly(methyl methacrylate) beads [47], 52.61 and 24.2 mg/g,respectively, the as-prepared CuI sample shows promising poten-

tial for industrial application as adsorbent for Cd (II). It shouldbe noted that the CuI cauliflower can be easily separated fromwater and efficiently recycled by rinsing with EDTA (abbr. Y) solu-tion (shown in Fig. 6B). Almost 99% of the adsorbed Cd (II) was

1026 Y. Jiang et al. / Materials Science and Engineering B 176 (2011) 1021– 1027

n ben

ratdneeacitaT(tC

Scheme 3. Possible reaction mechanism of the coupling reaction betwee

ecovered after the first cycle. The desorption efficiencies were 97%nd 95%, respectively, for the next two cycles. During the adsorp-ion/desorption process, the percentage of removal was slightlyecreased, which may be due to the incomplete release of theanoparticles during washing with water and the following regen-ration with Y. Why does the as-prepared CuI sample show suchxcellent removal capacity for Cd (II)? In our case, I− behaves asctive adsorption sites, and the adsorption should be ascribed to ahemical one. During the process of the adsorption, the high affin-ty of Cd (II) to I− results in the formation of stable CdI42−, induceshe great decrease of Cd2+ in the solution, and therefore makes thes-prepared CuI sample show high removal capacity for Cd (II).

his is quite different from physical adsorption for removing CdII) [48,49]. In order to decomplex the stable CdI42− and releasehe reactive adsorption sites, other ligands with higher affinity tod (II) should be utilized. It is well-known that the steady con-

Fig. 5. The FESEM and high-magnification im

zylamine and iodobenzene catalyzed by the as-prepared CuI cauliflower.

stant of CdY2−, 4.0 × 1016, is significantly larger than that of CdI42−,2.6 × 105. So after the addition of Y to the adsorption system, CdY2−

is produced, I− is released, and reactive adsorption sites are reacti-vated. The overall chemical adsorption/desorption process can beformulated based on chemical adsorption mechanism and shownas follows:

CuI � Cu+ + I− (3)

4I− + Cd2+ � CdI42− (4)

CdI42− + Y4− � CdY2− + 4I− (5)

Cu+ + I− � CuI (6)

ages of the commercial CuI powder.

Y. Jiang et al. / Materials Science and Eng

3210

20

40

60

80

100

120B

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orpt

ion

Cycle number

10008006004002000

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40

80

120

160 AC

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ig. 6. (A) Adsorption isotherm of Cd (II) on the as-prepared CuI cauliflower at 25 ◦C.B) Adsorption/desorption cycles.

. Conclusions

In summary, we have succeeded in synthesizing CuI cauliflowersing green route. Here, ampicillin plays dual roles, reducing andorphology-directing agents, in the formation of the product. The

rocedure that we describe here, in comparison with the reportedrotocols, is a green, environment-friendly, efficient, and directne-step process for the preparation of CuI cauliflower. Interest-ngly and importantly, the as-formed CuI cauliflower possessesigh specific surface area, which endows it with excellent cat-lytic activity and high adsorption capacity, as demonstrated hereor applications as catalyst for coupling reaction with remarkablectivity and adsorbent for Cd (II) with high removal capacity.

cknowledgments

The authors are grateful to the National Natural Science Foun-ation of China (No. 21071047), the Program for Science &

[

[

[

ineering B 176 (2011) 1021– 1027 1027

Technology Innovation Talents in Universities of Henan Province(2011HASTIT010), and the Henan Provincial Natural Science Foun-dation of China (092300410196) for their financial support.

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