PcMa

MD

a

ARRAA

KHMnPS

1

rCmamm[

tNraadlpu

h0

Applied Catalysis A: General 475 (2014) 477–486

Contents lists available at ScienceDirect

Applied Catalysis A: General

jou rn al hom ep age: www.elsev ier .com/ locate /apcata

alladium supported on zinc oxide nanoparticles: Synthesis,haracterization, and application as heterogeneous catalyst forizoroki–Heck and Sonogashira reactions under ligand-free and air

tmosphere conditions

ona Hosseini-Sarvari ∗, Zahra Razmi, Mohammad Mehdi Doroodmandepartment of Chemistry, Shiraz University, Shiraz 71454, Islamic Republic of Iran

r t i c l e i n f o

rticle history:eceived 18 November 2013eceived in revised form 25 January 2014ccepted 4 February 2014vailable online 12 February 2014

a b s t r a c t

In this paper, a novel Palladium (Pd) supported on ZnO nanoparticles was readily synthesized and char-acterized by using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electronmicroscopy (SEM), X-ray photoelectron spectroscopy (XPS), and BET specific surface area measurement.The total amount of palladium particles on ZnO was determined by induced coupled plasma (ICP) anal-ysis and atomic absorption spectroscopy (AAS) which is 9.8 wt%. Percentage of accessible Pd as active

eywords:eterogeneous Catalysisizoroki–Heck Reaction

anoparticlesd/ZnOonogashira reaction

catalyst is also estimated to 2.72% based on the thermogravimetric (TG) analysis. Nano Pd/ZnO was foundas a new, novel, and excellent heterogeneous catalyst for ligand-free C–C bond formation through theMizoroki–Heck and Sonogashira reactions under air atmosphere without the use of any Ar or N2 flow.The catalyst can be recovered and recycled several times without marked loss of activity.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

It is well-known that catalytic Mizoroki–Heck and Sonogashiraeactions are two powerful methodologies for the generation of–C bonds particularly in the coupling of aryl halides with ter-inal alkenes and alkynes for the generation of di-substituted

lkenes and alkynes derivatives. These compounds are importantaterials for the production of natural products, biologically activeolecules, fine chemicals, pharmaceuticals, and useful materials

1–4].Among the various coupling reactions, the Mizoroki–Heck reac-

ion is generally carried out in polar solvents such as DMF,,N-DMAC, CH3CN, and phosphine ligands [5], which are normally

equired to stabilize catalytically active palladium species. This canlso be achieved by employing salt additives such as ammoniumnd phosphonium chlorides and bromides [6]. To overcome therawbacks of the using phosphin ligands, ligand-free Heck ary-

ation conditions are clearly on the rise in view of industrial andharmaceutical needs. Developing a new green solvent systemnder ligand-free condition for the C–C bond formation is highly

∗ Corresponding author: Tel.: +98 711 6137169; fax: +98 711 6460788.E-mail address: hossaini@shirazu.ac.ir (M. Hosseini-Sarvari).

ttp://dx.doi.org/10.1016/j.apcata.2014.02.002926-860X/© 2014 Elsevier B.V. All rights reserved.

desirable. For this reason Pore et al. [7] in 2012 reported a novelhydrophobic fluorous ionic liquid approach for the ligand-free Heckreaction. Also this approach showed some advantages, the methodwas laborious (using 2 mmol triethyl amine as organic base). Bha-gat et al. [8] also reported a methodology for solvent-free one-potHeck reaction catalyzed by novel palladium(II) complex-via greenapproach, but this method suffered from difficulty and long reac-tion time of catalyst formation.

Pd-catalyzed reactions are undesired in the synthesis of phar-maceutically active compounds and bioactive molecules becauseof the high cost of this metal. Therefore, the efficient recovery ofthe catalyst, good reusability, minimum possibility for leaching ofthe metal species, and minimum uses of Pd catalysts remained ascientific challenge of economic and environmental perspective.For these reasons, palladium nanoparticles, colloidal palladiumspecies, and heterogeneous palladium catalytic systems on a sup-port (including organic, polymers, or inorganic supports) have beenmade [9–12]. However, these heterogeneous palladium catalystsoften suffer from problems such as low catalytic efficiency degra-dation, difficult synthetic procedures [9l], and the leaching of the

metal species [10f].

On the other hand, for another important cross coupling reactionnamely Sonogashira reactions, the use of phosphine ligands for lessreactive bromide and chloride substrates under new green solvent

4 Cataly

stfsparsCBbffa

wtfimhwbsakZa(aatwn�Spitw

afwpCt

2

2

ATgdawCidIPuL

78 M. Hosseini-Sarvari et al. / Applied

ystems is common. Recently much attention has been attracted tohe use of copper complexes and salts as the catalysts or cocatalystsor the Sonogashira coupling reaction [13]. These systems actuallyuffer from huge drawbacks such as formation of homo-couplingroducts, use of organic solvent, high temperature, phosphine lig-nds, and oxidizing agent or air [14]. For example Huang et al. [15],eported an aqueous phase synthesis of palladium tripod nano-tructures for Sonogashira coupling reactions under 2.5 mol % ofuI as cocatalyst in water without any homo-coupling products.ut in this method 5 mol % PPh3 and 2.0 equiv. KOH as a strongase was used. Borja and coworkers [16] reported a methodologyor Sonogashira coupling reaction under copper- and phosphine-ree conditions; however, this method was limited due to difficultynd longer reaction time of catalyst formation by a sol-gel process.

Among the important II–VI semiconductors, ZnO has beenidely studied because of its fundamental properties and poten-

ial uses in devices such as gas sensors, solar cells, resonators,eld-effect transistors, and as a catalyst [17]. In recent years, theodification of ZnO with noble metals such as Ag, Au, Pt, and Pd

as attracted significant attention [18]. As a noble metal, palladium,hose ionic radius (0.080 nm) is close to that of Zn2+ (0.074 nm), has

een widely used in the industry catalysis, especially for methanolynthesis [19]. However, modifying ZnO with palladium has beenpplied in the area of catalytic reaction [19,20]. To the best of ournowledge, there are a few studies on C–C coupling reactions withnO modified by palladium [21,22]. Kim and Choi [21] reported

method for preparing ZnO-supported Pd (Pd/ZnO) and pd-MM = Cu, Ni, and Ag) nano particles (Pd-M/ZnO) by �-irradiation,nd their catalytic efficiencies were evaluated in hydrogenationnd Suzuki reactions. Also, these catalysts showed some advan-ages, but either the method for the preparation of the catalystas laborious (using large amount of nano powder ZnO, usingitrogen gas to remove oxygen from the reaction vessel, using the-ray irradiation, and using organic solvent such as MeOH) or theuzuki C–C coupling reactions were limited only to iodohalides andhenylboronic acid and EtOH as solvent. Indeed, the molar ratio of

odohalid: phenylboronic acid: Pd/ZnO catalyst was 1:2:1.3, andhe loaded amount of the palladium on ZnO was estimated to 19.4t%.

Herein, in this paper, we focus on preparation and full char-cterization of Pd/ZnO, which appears to be highly active catalystor the Mizoroki–Heck and Sonogashira reactions. The goal of theork is the synthesis of Pd/ZnO by co-precipitation method withalladium supported on ZnO (palladium loading is 9.8 wt%) for the–C bond formations under ligand-free and air atmosphere condi-ions.

. Experimental

.1. Materials and instruments

Chemical materials were purchased from Fluka, Aldrich, Alfaesar, and Merck. The progress of the reactions was followed withLC using silicagel SILG/UV 254 plates or by GC using a Shimadzuas chromatograph (GC-10A) instrument with a flame ionizationetector using a column of 15.0% carboxwax 20.0 M chromosorb-wcid-washed 60–80 Å mesh size diameter. Evaporation of solventsas performed at reduced pressure, with a Buchi rotary evaporator.olumn chromatography was carried out on short columns of sil-

ca gel 60 (70–230 Å mesh size diameter) in glass columns (2–3 cmiameter) using 15–30 g of silica gel per one gram of crude mixture.

R spectra were run on a Shimadzu FTIR-8300 spectrophotometer.ower X-ray diffract meter with Cu K� (� = 1.54178 Å) radiation wassed. The morphology of the products was determined by usingeica Cambridge, model s360, version V03.03. Scanning electron

sis A: General 475 (2014) 477–486

microscopy (SEM) was performed at accelerating voltage of 25 kV.The amount of palladium nanoparticles supported on ZnO wasmeasured by inductively coupled plasma (ICP, Varian, Vista-pro)and atomic absorption spectroscopy (AAS, Varian Model SpectraAA 220 [Mulgrave, -Vic, -australia]). The size of the synthesizednanoparticles was also confirmed by a Philips CM10 TEM instru-ment. X-ray photoelectron spectroscopy (XPS) measurements wereconducted with a XR3E2 (VG Microtech) twin anode X-ray sourceusing AlK� = 1486.6 eV). A lab-made thermogravimetric analyzer(TGA) [23] was also adopted for studying both the interactionbehavior of CO (Linde, 99.99%) as a selective probe molecule withpalladium nanoparticles and thermal stability of Pd-supportedZnO nanoparticles after interacted with CO molecules. The specificsurface areas (SSABET; [m2 g−1]) of the nanopowders were deter-mined with the nitrogen adsorption measurement, applying theBET method at 77 K (BELsorp-mini II). The porous structural param-eter used in this paper was taken from Barret–Joyner–Halenda(BJH) data. 1H and 13CNMR spectra were obtained on a Bruker DPX250 MHz instrument.

2.2. Preparation of Pd/ZnO nanoparticles

Pd/ZnO catalyst was prepared by coprecipitation (CP) method.To a mixture of palladium nitrate (0.027 g/mL) and zinc nitrate(0.267 g/mL) solutions, aqueous solution of sodium carbonate (1 M)was added at room temperature to produce a final pH of 8. Thenit was aging for 2 h at 70–80 ◦C and the precipitates were filtered,washed several times with distilled water and absolute ethanol,dried at 80 ◦C overnight and then calcinated at 723 K for 2 h.

2.3. General procedure for Mizoroki–Heck coupling reaction ofaryl halides with styrene

A mixture of arylhalide (1 mmol), styrene (1 mmol), K2CO3(1 mmol), and nano Pd/ZnO (0.009 g, which contains 832 × 10−8

mol% of Pd which was determined by ICP) in H2O (1 mL) was placedin a 25 mL round bottom flask. In the case of the substrates whichare insoluble in water, a mixture of H2O/EtOH (1:1) was used assolvent. The mixture was stirred at 90 ◦C. After the reaction wasfinished, the reaction mixture was cooled to the room tempera-ture, diluted with ethyl acetate (5 mL), and the slurry was stirredat room temperature to ensure removal of the product from thesurface of the catalyst. Then it was centrifuged to separate the cata-lyst. The centrifugate was washed with water (2 × 5 mL), dried overanhydrous sodium sulfate, further concentrated under reducedpressure, and purified by column chromatography on silicagel togive the desired product.

2.4. General procedure for Mizoroki–Heck coupling reaction ofaryl halides with ethyl acrylate

To a 25 mL round bottom flask with a mixture of arylhalide(1 mmol), ethyl acrylate (1 mmol), K2CO3 (1 mmol), and nanoPd/ZnO (0.009 g), 1 mL of DMF was added. In the case of the sub-strates which are insoluble in water, a mixture of H2O/EtOH (1:1)was used as solvent. The resulting solution was stirred at 100 ◦Cin an oil bath. After the reaction was finished, it was cooled tothe room temperature, and DMF was removed under reducedpressure. The residue was diluted with ethyl acetate (5 mL), and

centrifuged to separate the catalyst. The centrifugate was washedwith water (2 × 5 mL), dried over anhydrous Na2SO4, filtered, andconcentrated. Further purification was achieved by column chro-matography.

Catalysis A: General 475 (2014) 477–486 479

2

(wswTa

od

2m

mtiwasAatlwidN

3

3

nTa

spadsvaw(scftpoatdit

Fno

M. Hosseini-Sarvari et al. / Applied

.5. General procedure for Sonogashira reaction

A mixture of arylhalide (1 mmol), phenylacetylene (1 mmol), CuI2 mol%), K2CO3 (1 mmol), and nano Pd/ZnO (0.009 g) in H2O (1 mL)as stirred at 90 ◦C for the appropriate time. In the case of the sub-

trates which are insoluble in water, a mixture of H2O/EtOH (1:1)as used as solvent. The progress of the reaction was monitored by

LC or GC. The separation of the catalyst from the reaction mixturend the work-up process is similar to the section 2.3.

All compounds were known and characterized by comparisonf their physical and spectroscopic data with the data alreadyescribed in the literature.

.6. General procedure for studying the interaction of CO as probeolecule with Pd-supported ZnO

To study the interaction between CO as a selected probeolecule with ZnO and Pd/ZnO nanoparticles, a constant quan-

ity (0.50 mg) of the synthesized nanoparticles was individuallynserted to the analyzing cell of the TG analyzer. The sample

as then heated to ∼120 ◦C inside N2 atmosphere with temper-ture ramp of ∼5.0 ◦C min−1 to desorb any previously adsorbedpecies such as water molecule or any volatile organic compounds.fter aging the sample at this temperature for ∼10 min, it wasnnealed to the room temperature with temperature ramp equalo -2.0 ◦C min−1, followed by changing the atmosphere of the ana-yzing cell from N2 to CO via purging a laminar flow of CO gas

ith 20 mL min−1 flow rate. Also, the thermal stability of the CO-nteracted Pd/ZnO nanoparticles was studied using the TG analyzeruring enhancing the temperature of the sample with 2.0 ◦C min in2 atmosphere.

. Results and Discussion

.1. Characterization of Pd/ZnO nanoparticles

In order to evaluate the content of Pd supported on ZnOanoparticles, the catalyst was analyzed by ICP and AAS analyzers.he results revealed that, the content of Pd-doped on ZnO was 9.84nd 9.80% (w/w) as measured by ICP and AAS, respectively.

Figure 1, shows the XRD patterns of pure and 9.84 wt% Pdupported on ZnO nanoparticles calcined at 723◦K. All diffractioneaks could be indexed to two crystalline phases. The XRD featuresppeared at 2� = 31.8◦, 34.5◦, 36.3◦, and 47.6◦, which were due to theiffractions of (100), (002), (101), and (102) planes of the wurtzitetructure ZnO (JCPDS Card File No. 36-1451), while the broad andery small peak (due to the low content of Pd) appear at a bragg’sngle of 41.2◦ originated from the diffraction of (111) planes, agreeell with the face-centered cubic (fcc) morphology of palladium

JCPDS Card File No. 05-0681), respectively. The diffraction inten-ity of Pd-doped ZnO is not as strong as that of the pure sample. Thisan be attributed to the effect of Pd doping. No characteristic peaksrom other impurities are detected in the patterns demonstratinghat the sample has high phase purity. It suggests that the obtainedroduct is Pd/ZnO sample. Furthermore, the average crystallite sizef the unmodified and 9.84 wt% palladium-modified ZnO, evalu-ted by the Scherre’r formula [24], are about 21–23 nm, indicatinghat the ZnO crystal does not change a little before and after palla-ium modification. Calculating the size particles of Pd by XRD was

mpossible because of very small amounts of Pd loaded on ZnO sohe peak of Pd(0) is not strong.

The XRD results are in agreement with observations from TEM.rom Figure 2a and 2b the average particle size of nano ZnO andano Pd/ZnO was shown to be ∼25 nm. As can be seen, the shapef nanoparticles is spherical and cubic at random. The size of nano

Figure 1. XRD pattern of (a) nano ZnO and (b) 9.84 wt% of Pd modified ZnO calcinedat 723 k.

ZnO which was determined by TEM is in agreement with the resultsobtained from XRD.

From the SEM image of nano Pd/ZnO in Figure 2c, we couldobserve the morphology of the Pd/ZnO nanoparticles, composedof spherical and cubic particles. The morphology showed rather adispersion of particles, with diameters ranging between 20–25 nm.While the SEM image provide morphology and estimated particlesizes, TEM image can reveal internal structure and a more accuratemeasurement of particle size and morphology. It should be notedthat, due to various reasons such as the same morphology and sizedistribution of each ZnO and Pd nanoparticles, partially the samecontrast of the electron beam through the ZnO and Pd nanoparti-cles during the TEM analysis, and finally, due to the phenomenonsuch as relative coagulation of the synthesized nanoparticles, nosignificant difference was observed between the morphology andstructure of ZnO and Pd nanoparticles during characterization byTEM (Figure 2c), even after enhancing the contrast by Au sputtering.

The surface area, average pore volume, and pore size of nanoPd/ZnO sample is given in Table 1. The diameter of particles canbe calculated by dBET 6/(SSABET × �samples 26 nm, where �samples

are the density of ZnO (�ZnO = 5.61 g/cm3) [25,26] and the densityof palladium (�Pd = 12.02 g/cm3) [27], which was compared withthe average crystalline sizes (dXRD ave = 23 nm) calculated by the

480 M. Hosseini-Sarvari et al. / Applied Cataly

Figure 2. (a) TEM image of ZnO; (b) TEM and (c) SEM images of the nano Pd/ZnO.

Table 1Results of BET surface area measurements for nano Pd/ZnO.

Surface area BET surface area(m2.g−1)BJH adsorptioncumulative surfacearea of pores (m2.g−1)

40.6138.18

Pore volume Single point adsorptiontotal pore volume ofpores (cm3.g−1)BJH adsorptioncumulative volume ofpores (cm3.g−1)

0.24

0.19

Pore size Mean pore diameter(nm)Pore size distribution(nm)

24.361.88

sis A: General 475 (2014) 477–486

fundamental parameter approach (Rietveld method) [24] basedon the half-maximum widths of Sherrer’s equation.

It is well known that material surface composition and chemicalstates are very important through the process of catalytic reaction.The surface composition and chemical states can be determinedby means of XPS spectrum according to the characterizing bindingenergies of different elements on material surfaces [28]. The XPSspectra of 9.8 wt% palladium modified ZnO was obtained. Figure 3shows the Pd3d, Zn2p, and O1s XPS spectra. From Figure 3a, it couldbe seen that the XPS signals were weak due to its low content ofpalladium, and the binding energy (BE) of the Pd3d5/2 XPS peak wasat about 335.5 eV. In general, the binding energy of Pd(3d5/2)

+2 isabout 337.0 eV according to the binding energy handbook of theXPS instrument [29]. Thus, the binding energy of Pd3d5/2 appearedat 335.5 eV, indicating that Pd exists mainly as the form of zero-valence Pd on ZnO surface. From Figure 3b, it can be seen thatthe peak position of Zn 2p3/2 in Pd/ZnO sample is about 1022.0 eVindicating that Zn is in the formal Zn+2 valance state [30]. The XPSspectrum of O1s is shown in Figure 3c. The peak centered at 530.2 eVis closely associated with the lattice oxygen (OL) of ZnO, that it isattributed to the contribution of Zn–O in ZnO crystal lattice [31].The peak at about 532.5 eV is attributed to the oxygen of surfacehydroxyl (OH) groups resulting from the chemisorbed water [32].

The FTIR spectrum of nano Pd/ZnO in KBr matrix is shown in(Figure 4). There is a broad band at 3425 cm−1 corresponding tothe vibration mode of water OH group indicating the presence ofsmall amount of water adsorbed on the ZnO nano crystal surface.The band at 1650 cm−1 is due to the OH bending of water. Theadsorption band at 460 cm−1 is the stretching mode of ZnO [17f].

In this study, to estimate the quantity of Pd-incorporated ZnOcrystal as well as to determine the percentage of accessible Pdas active catalyst, TG analyzer was selected as detection systemaccording to the recommended procedure. Figure 5 clearly showsthe trace (diagram of weight percentage vs. time) for each ZnOand Pd/ZnO nanoparticles. As clearly shown, the quantity of COinteracted with Pd was easily estimated according to the differencebetween the weight percentage of CO interacted with each ZnO andPd/ZnO nanoparticles, which was estimated to 4.3 ± 0.1% (Figure 5).For further reliability about this value, the thermal stability ofPd/ZnO nanoparticles was also investigated in detail. According tothe thermogram (Figure 6), a significant weight loss to 1.6 ± 0.1%at temperature to ∼95 ◦C is correlated to the desorption of H2O,organic species, and the CO-adsorbed Pd/ZnO. Whereas the weightloss (4.2 ± 0.1%) at ∼ 170 ◦C clearly points to the percentage of COinteracted with the Pd nanoparticles, which is in good agreementwith the result obtained from the trace (Figure 5). Considering pal-ladiumocene, i.e., Pd(CO)6 as thermodynamically stable species,this result simply points to the percentage of accessible Pd, whichis estimated to be 2.72 ± 0.01%. Consequently, from total amount ofPd, supported on ZnO crystal (9.84%), analyzed by the ICP and AAS,∼7.12% has been incorporated with the ZnO crystal and ∼2.72%is accessible for playing a role as catalyst during the synthesis oforganic compounds.

3.2. Catalytic application of nano Pd/ZnO

3.2.1. Mizoroki–Heck coupling reaction catalyzed by nano Pd/ZnOAfter successful preparation and characterization of nano

Pd/ZnO, its catalytic activity was first examined in theMizoroki–Heck reactions. This choice was based on the factthat such a reaction provides a powerful tool for the C–C bondformation from arylhalides and alkenes.

An attempt to develop C–C bond formations by Heck reaction,the cross coupling of 1-(4-bromophenyl)ethanone (1 mmol) andstyrene (1 mmol) in the presence of nano Pd/ZnO, was studiedin various parameters such as amount of catalyst, base, and

M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486 481

Figure 3. XPS spectra of (a) Pd3d, (b) Zn2p, and (c) O1s spectrum of 9.84 wt%, nanoPd/ZnO.

Figure 4. FTIR spectrum of nano Pd/ZnO.

Figure 5. Trace showing the interaction of CO as molecular probe with ZnO andPd/ZnO nanoparticles.

Figure 6. Thermogram showing the interaction of CO as molecular probe withPd/ZnO nanoparticles at N2 atmosphere.

solvent. The experimental results are summarized in Scheme 1,and Tables 2–4.

The results of the initial screen for the amounts of nano Pd/ZnOare displayed in Table 2. As it can be seen from entries 1–5, byincreasing the amount of catalyst from 0.0005 to 0.01 g, the yield ofdesired product increased, and the reaction time decreased. Therewere no differences in yield and reaction time between 0.009 g(entry 4) and 0.01 g (entry 5) of the catalyst. Therefore, because oflower amount of Pd loading, we chose 0.009 g of the nano Pd/ZnO

for further experiments.

Optimization with this catalyst was completed with differentbases and solvents. Based on the results outlined in Table 3, it is

Scheme 1. Mizoroki–Heck reaction of 1-(4-bromophenyl)ethanone and styren bynano Pd/ZnO catalyst.

482 M. Hosseini-Sarvari et al. / Applied Cataly

Table 2Optimization of the amount of nano Pd/ZnO in Heck reaction.a

Entry 1 2 3 4 5

Nano Pd/ZnO (g) 0.0005 0.001 0.005 0.009 0.01Time (h) 22 20 20 17 16Conversion %b 95 95 97 98 98

a Reaction conditions: 1-(4-Bromophenyl)ethanone (1 mmol), styrene (1 mmol),nano Pd/ZnO, H2O (1 mL), and K2CO3 (1 mmol) at 90 ◦C. b Determined by GC.

Table 3The effect of base on Heck coupling reaction.a

Entry Base mmol Time (h) Conversion %b

1 K2CO3 1 17 982 K2CO3 0.5 17 953 Na2CO3 1 20 984 K3PO4 1 18 985 KF.2H2O 1 20 906 KOH 1 18 957 Cs2CO3 1 20 708 NaOAC 1 20 65

a

n

eoatRstsupp2ao

rw(wcywttiih(

TT

a

nb

Reaction conditions: 1-(4-Bromophenyl)ethanone (1 mmol), styrene (1 mmol),ano Pd/ZnO (0.009 g), H2O (1 mL), and base at 90 ◦C.b Determined by GC.

vidence that K2CO3, Na2CO3, and K3PO4 are superior bases to thethers. Consequently, because K2CO3 is an inexpensive and readilyvailable inorganic base, we chose it for further investigations. Also,he selection of solvent was important for the present method.esults (Table 4) show that nano Pd/ZnO is a very efficient anduitable catalyst for such coupling reaction in water. We then triedhis reaction with catalyst in other solvent. The use of protic polarolvents, such as EtOH and ethane-1,2-diol, which are commonlysed in Pd catalyzed coupling reactions, were not effective in theresent method (Table 4, entries 2 and 3). Use of DMSO and DMFrovides long reaction times and the reactions were completed in2 and 19 h, respectively (Table 4, entries 4 and 5). Use of toluenend CH2Cl2 is not successful and only 10% conversion yields werebserved (Table 4, entries 7 and 8).

On the basis of the optimized reaction conditions, the couplingeactions between a range of aryl halides (I, Br, and Cl) and styreneere carried out in the presence of nano Pd/ZnO (0.009 g), K2CO3

1 mmol), and H2O (1 mL) at 90 ◦C. As shown in Table 5, aryl iodidesith a variety of electron-rich and electron-poor substituent pro-

eeded smoothly, giving the coupling products in good to excellentields. The reactivity of aryl bromides and chlorides with electron-ithdrawing substituent (Table 5, entries 8–11) was higher than

hose with electron donating substituent (Table 5, entry 7). Fur-hermore, steric hindrance due to the ortho substituents on the arylodides and chlorides affected the reaction progress. Thus 2-methyl

odobenzene and 1-chloro-2-(trifluoromethyl) benzene needed aigher reaction time than 4-substitued iodo and chloro benzenesTable 5, entries 5, 11).

able 4he effect of solvents on Heck coupling reaction.a

Entry Solvent Time (h) Conversion %b

1 H2O 17 982 EtOH 18 653 Ethane-1,2-diol 20 554 DMSO 22 905 DMF 19 956 Solvent free 20 207 Toluene 20 108 CH2Cl2 20 10

Reaction conditions: 1-(4-bromophenyl)ethanone (1 mmol), styrene (1 mmol),ano Pd/ZnO (0.009 g), solvent (1 mL), and K2CO3 (1 mmol) at 90 ◦C.b Determinedy GC.

sis A: General 475 (2014) 477–486

Then we tried to study the Heck reaction between aryl halidesand ethyl acrylate in order to show the originality and generality ofnano Pd/ZnO catalyst. By using the similar conditions for the Heckreaction described above in water as solvent, a trace amount of theproduct was observed. Therefore, we screened different solventsand bases. Finally we found that for this reaction by using DMF(1 mL) as solvent and K2CO3 (1 mmol) as base at 100 ◦C, after 3 h,98% of the desired product was obtained. Therefore, on the basisof optimized reaction conditions, the coupling between a range ofaryl halides and ethyl acrylate were carried out (Table 6).

Similar to the reaction between aryl halides and styrenethe reactivity of aryl bromides and chlorides with electron-withdrawing substituent (Table 6, entries 4,8–10) was higher thanthose with electron donating substituent (Table 6, entries 2,6).Sterically hindered aryl iodides and chlorides were also found toreact smoothly with ethyl acrylate providing excellent yields of thedesired products. (Table 6, entries 5, 11).

3.3. Sonogashira coupling reaction catalyzed by nano Pd/ZnO

Impressed with the results for the Mizoroki–Heck couplingreaction, we further explored the application of the nano Pd/ZnOcatalyst in another important C–C bond forming reaction, namelySonogashira cross-coupling of arylhalides with phenylacetylene. Asa model study, we first chose to study the effect of CuI salt on theefficiency of the nano Pd/ZnO catalyzed Sonogashira cross-couplingreaction of 1-(4-bromophenyl)ethanone and phenylacetylene.When the reaction was performed with 2 mol% of CuI and nanoPd/ZnO (0.009 g) in the presence of K2CO3 as a base in waterat 90 ◦C for 10h, the expected cross-coupling product of phenyl-ethynylbenzene was afforded in 98% yield (Table 7, entry 6).

For further optimization of the reaction conditions, thedecreasing yields of the coupling product was observed bydecreasing the added amounts of CuI and nano Pd/ZnO catalyst(Table 7, entries 1–5). For a blank test (Table 7, entry 7), a signifi-cantly lower 10% yield of the cross-coupling product was obtainedwhile the same reaction condition was carried out in the absenceof the nano Pd/ZnO catalyst. To evaluate the effect of the solvent(Table 7, entries 6, 8–12), results showed that H2O is the best choice.For comparison on the efficiency of base in this reaction (Table 7,entries 6, 13–15) K2CO3 was found to be the most effective andtherefore, 1 mmol of K2CO3 as an inexpensive and readily availableinorganic base was used in this study.

On the basis of the optimized reaction conditions, the couplingreactions between a range of arylhalides (I, Br, and Cl) and phenylacetylene were carried out in the presence of nano Pd/ZnO. Byusing this protocol, the coupling reaction of various functional-ized arylhalides with phenyl acetylene can give the correspondingarylated alkynes in good to excellent yields (Table 8). Similar toHeck reaction, it has been found that, aryliodieds with a varietyof electron-rich and electron-poor substituent proceeded, givingthe coupling products in good to excellent yields (Table 8, entries1–6). The reactivity of aryl bromides and chlorides with electron-withdrawing substituent (Table 8, entries 8–12) was higher thanthose with electron donating substituent (Table 8, entry 7). Inter-estingly, no significant steric effects were observed in this study,because sterically hindered ortho-substituted iodobenzene alsoprovided good yields with phenylacetylene (Table 8, entries 5, 12).

Finally, in order to show the merit of this catalytic method, wecompared our obtained results with similar data published by othergroups which used other catalysts (Table 9).

3.3.1. Catalyst recycling in Mizoroki–Heck and Sonogashirareactions

Having established the efficacy of the Pd/ZnO nanoparticles inthe Heck and Sonogashira coupling reactions, we then investigated

M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486 483

Table 5Mizoroki–Heck reaction of aryl halides with styrene.a

Entry X R Time (h) Yield %a

1 I H 17 952 I 4-OMe 24 953 I 4-NH2 22 954 I 4-NO2 10 1005 I 2-CH3 24 906 I 4-CH3 20 907 Br 4-OH 23 908 Br 4-CN 12 929 Br 4-COCH3 17 9810 Cl 4-CN 15 8511 Cl 4-CF3 10 10012 Cl 2-CF3 15 95

aReaction conditions: arylhalide (1 mmol), styrene (1 mmol), K2CO3 (1 mmol), Pd/ZnO (0.009 g), and H2O (1 mL) at 90 ◦C.b Isolated yield.

Table 6Mizoroki–Heck reaction of aryl halides with ethyl acrylatea.

Entry X R Time (h) Yield (%)b

1 I H 3.10 982 I 4-OMe 5 953 I 4-NH2 6 884 I 4-NO2 3 985 I 2-CH3 8 906 I 4-CH3 8 937 Br 4-OH 6 908 Br 4-CN 4 999 Br 4-COCH3 3.15 9810 Cl 4-CN 5 95

a mol),

taor

TS

a

11 Cl 4-CF3

12 Cl 2-CF3

Reaction conditions: arylhalide (1.0 mmol), ethyl acrylate (1.0 mmol), K2CO3 (1.0 m

he recyclability of the catalyst (Table 10). To clarify this issue, cat-lytic recycling experiments were carried out using a Heck reactionf 1-(4-bromophenyl)ethanone with styrene, and a Sonogashiraeaction of 1-(4-bromophenyl)ethanone with phenyl acetylene as

able 7onogashira cross-coupling reaction of 1-(4-bromophenyl)ethanone with phenylacetylen

Entry CuI:Pd/ZnO(mol%) : (g) Bas

1 0.5:0.0005 K2C2 0.5:0.001 K2C3 0.5:0.003 K2C4 0.5:0.005 K2C5 1:0.005 K2C6 2:0.009 K2C7 2:0 K2C8 2:0.009 K2C9 2:0.009 K2C10 2:0.009 K2C11 2:0.009 K2C12 2:0.009 K2C13 2:0.009 K3P14 2:0.009 Cs2

15 2:0.009 Na

Reaction conditions: 1-(4-Bromophenyl)ethanone (1 mmol), phenylacetylene (1 mmol), n

4 985 94

Pd/ZnO (0.009 g), and DMF (1.0 mL) at 100 ◦C.b Isolated yield.

model reactions. In both reactions, after completion for the firstreaction, the catalyst could be recovered from the reaction mixtureby diluting with EtOAc and centrifugation to separate the catalyst.The recovered catalyst was further washed with EtOAc and water

e.a

e Solvent Yield %b

O3 H2O 40O3 H2O 55O3 H2O 80O3 H2O 80O3 H2O 90O3 H2O 98O3 H2O 30O3 EtOH 80O3 DMSO 70O3 DMF 70O3 CH3CN 48O3 PHCH3 40O4 H2O 90CO3 H2O 89

2CO3 H2O 95

ano Pd/ZnO, CuI, solvent (1 mL), and base (1 mmol) at 90 ◦C for 7 h.b Isolated yield.

484 M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486

Table 8Sonogashira reaction of aryl halides with phenylacetylene.a

Entry X R Time (h) Yield %b

1 I H 7 982 I 4-OMe 8 953 I 4-NH2 10 954 I 4-NO2 5 1005 I 2-CH3 8 906 I 4-CH3 8 957 Br 4-OH 10 908 Br 4-CN 4 929 Br 4-COCH3 7 9810 Cl 4-CN 7 8511 Cl 4-CF3 5 10012 Cl 2-CF3 5 100

a Reaction conditions: arylhalide (1 mmol), phenylacetylene (1 mmol), K2CO3 (1 mmol), Pd/ZnO (0.009 g), and H2O (1.0 mL) at 90 ◦C.b Isolated yield.

Table 9Comparison of activity of different catalysts in the Heck and Sonogashira cross-coupling reactions.

Catalyst Reaction Conditions Time (h) Yield (%) Ref.

Hecka nano Pd/AT-Mont (0.07 mol%) CH3CN, NEt3, 82 ◦C 5 87 [9f]nano Pd/SiO2 (0.3 mol%) NMP, NaOAc, 150 ◦C 5 54a [9h]nano PdO/PS (1.0 mol%) H2O, KOH, 90 ◦C 20 14 [9g]nano Pd/NH2-SiO2 (0.05 mol%) DMF, K2CO3, 110 ◦C 2 95 [9l]nano Pd–Fe3O4 (1 mol%) DMF, NaOAc, 110 ◦C 24 91 [9j]nano Pd/ZnO (8.32 × 10−6 mol% of Pd) H2O, K2CO3, 90 ◦C 17 98 This study

Sonogashirab nano Pd/AT-Mont (0.07 mol%) CH3CN, NEt3, 82 ◦C 3 90 [9f]Pd-2QC-MCM-41 (0.02 g) NMP, piperidine, 80 ◦C 3 99 [9i]nano Pd-Fe3O4 (1 mol%) DMF, piperidine, 110 ◦C 24 91 [9j]Pd/MgLa (1.5 mol%) DMF, NEt3, 80 ◦C 10 90 [9k]

2CO3,C

afa

i

TR

nano Pd/ZnO (8.32 × 10−6 mol% of Pd) H2O, K

a Phenyl iodide and styrene as starting materials;b Phenyl iodide and phenyl acetylene as starting materials.

nd finally dried at 80 ◦C. A new reaction was then performed with

resh solvent and reactants under the same conditions. The resultsre shown in Table 10 and Figure 7.

The amount of palladium leaching of the catalyst was furthernvestigated by ICP and AAS. The result indicated that only 0.01%

able 10eusabilities of the Pd/ZnO catalyst.

Run Yield (%) 3a (Heck reaction)a

First 98

Second 98

Third 97

Fourth 95

Fifth 95

a The reaction was terminated after 17 h.b The reaction was terminated after 7 h.

uI, 90 ◦C 7 98 This study

wt of the Pd metal (the initial 9.84 w% Pd goes to 9.83 ww% after

5th cycle) is leached out from the catalyst surface after five cycles(for both Heck and Sonogashira reactions) (Table 10), which wasdetermined by ICP and ASS in both liquid phase and solid mate-rial. Furthermore, the filtered solution exhibited no reactivity to

Yield (%) 3b (Sonogashira reaction)b

9898979797

M. Hosseini-Sarvari et al. / Applied Catalysis A: General 475 (2014) 477–486 485

Ff

bdiprtait

tTwirb

7978–7984.

igure 7. Plot of the yields versus the reused cylces as a function of reaction timeor the a) Heck and b) Sonogashira reaction.

oth Heck and Sonogashira reactions. The absence of any obviousecrease leached metal in the filtrate suggests that high stabil-

ty of the heterogeneous catalyst and confirming that the catalyticrocess occurs on solid surface. The XRD pattern (Figure 8) of theecovered nano catalyst for Heck reaction after the 5th run showedhat the aggregation of the particles has not occurred significantly,nd the size of the particles were not disturbed notably in compar-son with the XRD picture of the catalyst before its application forhe reaction.

This observation confirms that nano Pd/ZnO are stable underhe reaction conditions and are not affected by the reactants.his could be attributed to the high stability of the nano Pd/ZnO,

hich behaves truly as a heterogeneous solid catalyst. Also,

n another experiment when the catalyst separated from theeaction mixture in the middle of the Heck reaction between 1-(4-romophenyl)ethanone and styrene (8 h) or Sonogashira reaction

Figure 8. XRD pattern of the nano Pd/ZnO used for the reaction of 1-(4-bromophenyl)ethanone with styrene in the Heck reaction, after the 5th run recyclingof the catalyst.

(3 h), respectively. The filtrate was further heated, no extra forma-tion of coupling product was observed via GC analysis even after20 h and 10 h for each reaction, respectively. These observationsconfirm that the reactions catalyzed by the nano Pd/ZnO are het-erogeneous in nature.

4. Conclusions

In conclusion, we have successfully prepared Pd supported onZnO nanoparticles as heterogeneous catalyst for Heck and Sono-gashiro cross coupling reactions, without using any ligands and Aror N2 flow. The results reported indicate that the behavior of palla-dium catalyst, in the C–C coupling reactions, can be modified usingan appropriate preparation method (coprecipitation technique).Moreover, the easy extractive recovery of the final product, and thesolid residue can be reused for several times, can be considered asstrong practical advantages of this method. This methodology couldprovide a facile, efficient, and environmentally friendly process forthe Heck and Sonogashira reactions because of its wide applicabil-ity to various substrates, the use of less toxic reagents, and mildreaction conditions. We believe that this type of nano crystallinemetal oxide finds excellent applications as active catalysts not onlyfor laboratory scale research but also for industrial applications.

Acknowledgments

We gratefully acknowledge the support of this work by the Shi-raz University and the Iran National Science Foundation (GrantNo.90003779) for financial support.

References

[1] (a) I. Paterson, R.D.M. Davies, R. Marquez, Angew. Chem., Int. Ed 40 (2001)603–607;(b) N.D.P. Cosford, L. Tehrani, J. Roppe, E. Schweiger, N.D. Smith, J. Anderson, L.Bristow, J. Brodkin, X. Jiang, I. McDonald, S. Rao, M. Washburn, M.A. Varney, J.Med. Chem 46 (2003) 204–206.

[2] (a) J. Li, A. Ambroise, S.I. Yang, J.R. Diers, J. Seth, C.R. Wack, D.F. Bocian, D. Holten,J.S. Lindsey, J. Am. Chem. Soc 121 (1999) 8927–8940;(b) K. Onitsuka, M. Fujimoto, N. Ohshiro, S. Takahashi, Angew. Chem. Int. Ed 38(1999) 689–692;(c) L. Brunsveld, E.W. Meijer, R.B. Prince, J.S. Moore, J. Am. Chem. Soc 123 (2001)

[3] (a) K.-T. Wong, C.C. Hsu, Org. Lett 3 (2001) 173–175;(b) O. Mongin, L. Porres, L. Moreaux, J. Merta, M. Blanchard-Desce, Org. Lett 4(2002) 719–722.

[4] R.R. Bader, P. Baumeister, H.U. Blaser, Chimia 50 (1996) 99–105.

4 Cataly

[

[

[

[

[

[[

[

[

[

[

[[

[

[[[[

[[

[

86 M. Hosseini-Sarvari et al. / Applied

[5] (a) G.Y. Li, Angew. Chem 113 (2001) 1561–1564;(b) D.K. Morita, J.K. Stille, J.R. Norton, J. Am. Chem. Soc 117 (1995) 8576–8581;(c) R. Martin, S.L. Buchwald, Acc. Chem. Res 41 (2008) 1461–1473.

[6] (a) T. Jeffery, Tetrahedron Lett 26 (1985) 2667–2669;(b) C. Amatorc, M. Azzabi, A. Jutand, J. Am. Chem. Soc 113 (1991) 8375–8384;(c) S.S. Prockl, W. Kleist, K. Kohler, Catal. Lett 125 (2008) 197–200.

[7] D.S. Gaikwad, Y.K. Park, D.M. Pore, Tetrahedron Lett 53 (2012) 3077–3081.[8] P. Karthikeyan, P.N. Muskawar, S.A. Aswar, P.R. Bhagat, S.K. Sythana, J. Mol.

Catal. A: Chem 358 (2012) 112–120.[9] (a) R. Narayanan, Molecules 15 (2010) 2124–2138;

(b) V. Farina, Adv. Synth. Catal 346 (2004) 1553–1582;(c) H.–U. Blaser, A. Indolese, F. Naud, U. Nettekoven, A. Schnyder, Adv. Synth.Catal 346 (2004) 1583–1598;(d) A. Zapf, M. Beller, Chem. Commun (2005) 431–440;(e) G. Zeni, R.C. Larock, Chem. Rev 106 (2006) 4644–4680;(f) M. Lamblin, L. Nassar-Hardy, J.-C. Hierso, E. Fouquet, F.–X. Felpin, Adv. Synth.Catal 352 (2010) 33–79;(g) B.J. Borah, D.K. Dutta, J. Mol. Catal. A: Chem 366 (2013) 202–209;(h) A. Ohtaka, T. Yamaguchi, T. Teratani, O. Shimomura, R. Nomura, Molecules16 (2011) 9067–9076;(i) A. Molnar, A. Papp, K. Miklós, P. Forgo, Chem. Commun (2003)2626–2627;(j) K. Komura, H. Nakamura, Y. Sugi, J. Mol. Catal. A: Chem 293 (2008)72–78;(k) J. Chung, J. Kim, Y. Jang, S. Byun, T. Hyeon, B.M. Kim, Tetrahedron Lett 54(2013) 5192–5196;(l) A. Cwik, Z. Hell, F. Figueras, Tetrahedron Lett 47 (2006) 3023–3026;(m) P. Veerakumara, M. Velayudham, K.–L. Lu, S. Rajagopala, Appl. Catal. A Gen455 (2013) 247–260.

10] (a) M. Al-Hashimi, A.C. Sullivan, J.R.H. Wilson, J. Mol. Catal. A 273 (2007)298–302;(b) H. Mahdavi, A. Zirakzadeh, J. Amani, React. Funct. Polym 67 (2007) 716–722;(c) M. Cai, Q. Xu, Y. Huang, J. Mol. Catal. A 271 (2007) 93–97;(d) T.K. Sau, C.J. Murphy, J. Am. Chem. Soc 126 (2004) 8648–8649;(e) T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Science 272(1996) 1924–1925;(f) P.-P. Fang, A. Jutand, Z.–Q. Tian, C. Amatore, Angew. Chem. Int. Ed 50 (2011)12184–12188.

11] (a) S.–D. Oh, M.–R. Kim, S.–H. Choi, J. Ind. Eng. Chem 14 (2008) 687–692;(b) L. Zhang, X. Wang, Y. Zhao, Z. Zhu, H. Fong, Mater. Lett 68 (2011)133–136;(c) Y.–W. Lee, S.–B. Han, A.R. Ko, H.–S. Kim, K.–W. Park, Catal. Commun 15(2011) 137–140.

12] (a) S.–J. Kim, S.–D. Oh, S. Lee, S.–H. Choi, J. Ind. Eng. Chem 14 (2008)449–456;(b) G.A. Molander, I. Shin, Org. Lett 15 (2013) 2534–2537;(c) G.A. Molander, S.L.J. Trice, S.M. Kennedy, Org. Lett 77 (2013) 8678–8688.

13] (a) S.S. Bag, R. Kundu, M, Das, J. Org. Chem. 76 (2011) 2332–2337;

(b) H.-J. Chen, Z.–Y. Lin, M.–Y. Li, R.–J. Lian, Q.–W. Xue, J.–L. Chung, S.–C. Chen,Y.–J. Chen, Tetrahedron 66 (2010) 7755–7761;(c) J. Mao, J. Guo, S. Ji, J. Mol. Catal. A 284 (2008) 85–88;(d) K.G. Thakur, E.A. Jaseer, A.B. Naidu, G. Sekar, Tetrahedron Lett 50 (2009)2856–2869.

[

[

sis A: General 475 (2014) 477–486

14] (a) C. Glaser, Ber. Dtsch. Chem. Ges 2 (1869) 422–424;(b) A.S. Hay, J. Org. Chem 27 (1962) 3320–3321;(c) P. Siemsen, R.C. Livingston, F. Diederich, Angew. Chem., Int. Ed 39 (2000)2632–2657.

15] Y.–T. Chu, K. Chanda, P.–H. Lin, M.H. Hung, Langmuir 28 (2012) 11258–11264.16] G. Borja, A. Monge-Marcet, R. Pleixats, T. Parella, X. Cattoen, M. Wong Chi Man,

Eur. J. Org. Chem 2012 (2012) 3625–3635.17] (a) V. Srikant, D.R. Clarke, J. Appl. Phys 83 (1998) 5447–5451;

(b) M. Hosseini-Sarvari, Z. Mardaneh, Bull. Chem. Soc. Jpn 84 (2011) 778–782;(c) M. Hosseini-Sarvari, H. Sharghi, S. Etemad, Helv. Chim. Acta 91 (2008)715–724;(d) M. Hosseini-Sarvari, Synth. Cummun 38 (2008) 832–840;(e) M. Hosseini-Sarvari, M. Tavakolian, New. J. Chem 36 (2012) 1014–1021;(f) M. Hosseini-Sarvari, Catal. Lett 141 (2011) 347–355.

18] (a) Y. Zheng, L. Zheng, Y. Zhan, X. Lin, Q. Zheng, K. Wei, Inorg. Chem 46 (2007)6980–6986;(b) J.–J. Wu, C.–H. Tseng, Appl. Catal. B 66 (2006) 51–57;(c) H. Zeng, P. Liu, W. Cai, S. Yang, X. Xu, J. Phys. Chem. C 112 (2008)19620–19624;(d) J.J. Dodson, L.M. Neal, H.E. Hagelin-Weaver, J. Mol. Catal. A. Chem 341 (2011)42–50.

19] (a) M.L. Cubeiro, J.L.G. Fierro, J. Catal 179 (1998) 150–162;(b) M.L. Cubeiro, J.L.G. Fierro, Appl. Catal. A 168 (1998) 307–322;(c) Y.–H. Chin, R. Dagle, J. Hu, A.C. Dohnalkova, Y. Wang, Catal. Today 77 (2002)79–88.

20] (a) N. Semagina, M. Grasemann, N. Xanthopoulos, A. Renken, L. Kiwi-Minsker,J. Catal 251 (2007) 213–222;(b) S. Liu, K. Takahashi, H. Eguchi, K. Uematsu. Catal. Today 129 (2007) 287–292.

21] M.–R. Kim, S.–H. Choi, J. Nanomater 7 (2009) 302919.22] M. Wagner, K. Kohler, L. Djakovitch, S. Weinkauf, V. Hagen, M. Muhler, Topics

in Catalysis 13 (2000) 319–326.23] M.M. Doroodmand, Synthesis, Characterization, and Some Analytical Appli-

cation of Carbon Nanostructures and Metal Nanoparticles, Thesis, ShirazUniversity, Shiraz, Iran, 2009.

24] Q.H. Zhang, L. Gao, J.K. Guo, Appl. Catal. B 26 (2000) 207–215.25] C. Liewhiran, S. Phanichphant, Current, Appl, Phys. 8 (2008) 336–339.26] T. Tani, L. Mädler, S.E. Pratsinis, J. Nanopart. Res 4 (2002) 337–343.27] J.C. Moreno-Maryn, I. Abril, S. Heredia-Avalos, R. Garcia-Molina, Nucl. Instrum.

Methods Phys. Res. Sect. B 249 (2006) 29–33.28] L. Jing, Z. Xu, X. Sun, J. Shang, W. Cai, Appl. Surf. Sci. 180 (2001) 308–314.29] J. Liqiang, W. Dejun, W. Baiqi, L. Shudan, X. Baifu, F. Honggang, S. Jiazhong, J.

Mol. Catal. A. Chem 244 (2006) 193–200.30] (a) L. Jing, Z. Xu, J. Shang, X. Sun, W. Cai, H. Guo, Mater. Sci. Eng. A 332 (2002)

356–361;(b) W. Peng, S. Qu, G. Cong, Z. Wang, Cryst. Growth. Des 6 (2006) 1518–1522;(c) N.S. Ramgir, D.J. Late, A.B. Bhise, M.A. More, I.S. Mulla, D.S. Joag, K.Vijayanohanan, J. Phys. Chem. B 110 (2006) 18236–18242;(d) W. Lu, S. Gao, J. Wang, J. Phys. Chem. C 112 (2008) 16792–16800.

31] (a) Y. Du, M.S. Zhang, J. Hong, Y. Shen, Q. Chen, Z. Yin, Appl. Phys. A: Mater. Sci.Process 76 (2003) 171–176;(b) Y.H. Zheng, C.Q. Chen, Y.Y. Zhan, X.Y. Lin, Q. Zheng, K.M. Wei, J.F. Zhu, Y. Zhu,J. Inorg. Chem 46 (2007) 6675–6682.

32] B.J. Coppa, R.F. Davis, R. Nemanich, J. Appl. Phys. Lett 82 (2003) 400–402.