the application of ultrasound radiation to the synthesis of nanocrystalline

The application of ultrasound radiation to the synthesis of nanocrystallinemetal oxide in a non-aqueous solvent Department of Chemistry, Kanbar Laboratory for Nanomaterials, Nanotechnolo

The application of ultrasound radiation to the synthesis of nanocrystalline

metal oxide in a non-aqueous solvent

Department of Chemistry, Kanbar Laboratory for Nanomaterials, Nanotechnology Research Center, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel

a r t i c l e i n f o

Article history:

Received 17 March 2009

Received in revised form 14 May 2009

Accepted 15 May 2009

Available online 22 May 2009

Keywords:

Sonochemistry

Nanoparticles

Non-aqueous solvent

Microwave

a b s t r a c t

Highly crystalline metal oxide nanoparticles of TiO2, WO3, and V2O5were synthesized in just a few min-utes by reacting transition metal chloride with benzyl alcohol using ultrasonic irradiation under argon atmosphere in a non-aqueous solvent The sonochemical process was conducted at a relatively low tem-perature, 363 K A unique crystallization process of these nanoparticles has been observed and character-ized by powder X-ray diffraction (PXRD), high resolution scanning electron microscopy (HRSEM), and BET The particles’ size and shape measured from HRSEM reveal ‘‘quasi” zero-dimensional, spherical TiO2particles in the range of 3–7 nm The V2O5particles have a ‘‘quasi” one-dimensional ellipsoidal mor-phology, with lengths in the range of 150–200 nm and widths varying between 40 and 60 nm The WO3

particles were obtained as ‘‘quasi” two-dimensional platelets with square shapes having facets ranging from 30 to 50 nm The thickness of these platelets was between 2 and 7 nm The mechanism of the reac-tions leading to these three metal oxide nanoparticles in a non-aqueous system is substantiated by Nuclear Magnetic Resonance (NMR), and Electron Spin Resonance (ESR)

Ó 2009 Elsevier B.V All rights reserved

1 Introduction

Among the different synthesis approaches developed during the

last few years, non-aqueous or non-hydrolytic processes were

par-ticularly successful with respect to achieving control of crystallite

size, shape, and hence over dimensionality The synthesis of metal

oxide nanoparticles in an organic solvent without the addition of

water avoided the major problems of aqueous sol–gel methods,

which are based on the hydrolysis of halide precursors, fast

reac-tions, and amorphous products Therefore, non-aqueous solution

routes to create metal oxide are valuable alternatives to the known

aqueous sol–gel process The pioneering work of Niederberger

et al has shown that benzyl alcohol as a solvent enabled the

syn-thesis of a long list of nanosized compounds, especially metal

oxi-des [1–5] Niederberger carried out the reactions under

solvothermal conditions The titanium was heated to 40 °C

be-tween 7 and 14 days For the vanadium and tungsten oxide, the

aging time was 48 h at 100 °C and 120 °C, respectively More

re-cently, he demonstrated that using benzyl alcohol as a solvent

for a solvothermal reaction can be conducted under microwave

radiation, thus saving energy and time[6] In the microwave

reac-tion other metal oxides were prepared, but not titanium, vanadium

or tungsten oxides

The growing number of publications dealing with ultrasonic irradiation gives an idea about the great potential of the method because of its simplicity and efficiency The sonochemical synthesis of metal oxide nanoparticles, as well as many other nanoparticles, has been reported and reviewed It was natural that the non-aqueous process would be extended and performed by using ultrasonic waves The application of sonochemistry allows good yields, high crystallinity of the products, and is more efficient and homogeneous Sonochemistry affords an immense reduction

of the reaction time, from days to minutes

Herein we present a novel, simple sonochemical one-step pro-cess using metal chloride precursors (TiCl4, WCl6, and VOCl3), and obtaining the corresponding nanosized metal oxides (TiO2, WO3, and V2O5) as products In all three reactions, benzyl alcohol

is simultaneously used as a solvent and a ligand, and as mentioned above, the reaction is completed within a few minutes All the spe-cific nanostructured metal oxides have been intensively studied as

a result of their outstanding chemical and physical properties For example, titanium oxides are of interest for applications as gas sen-sors[7–11], catalysts[12], photo catalysts[13–16], and photovol-taic cells[17–22] Vanadium oxides are applied in catalysis[23] and electrochemistry[24], and tungsten oxides have been investi-gated for electrochromic device technology[25]

The synthesis of the metal oxide nanoparticles involved disper-sion of the precursors in benzyl alcohol (Scheme 1), followed by heating them in a sonicator of 60% amplitude, with the collapse

of acoustic the bubble resulting in the shorter reaction time

1350-4177/$ - see front matter Ó 2009 Elsevier B.V All rights reserved.

* Corresponding author.

E-mail address: gedanken@mail.biu.ac.il (A Gedanken).

Contents lists available atScienceDirect

Ultrasonics Sonochemistry

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / u l t s o n c h

2 Experimental section

2.1 Materials

Titanium (IV) chloride (99.9%), Vanadium (V) oxychloride

(99.99%), tungsten (VI) chloride (powder 99.9%) and benzyl alcohol

(99.8%, anhydrous) were obtained from Aldrich These chemicals

were used without further purification

2.2 Synthesis of the metal oxide nanoparticles

In a typical preparation, the transition metal chloride, 0.5 mL of

TiCl4(4.5 mmol), 400 mg of WCl6(1 mmol), and 0.5 mL of VOCl3

(5.3 mmol), was slowly added to 20 mL of benzyl alcohol

(0.193 mol) under vigorous stirring at room temperature All the

materials were introduced into the sonication cell under inert

atmosphere in a glove box The reaction mixture was sonicated

un-der argon–hydrogen atmosphere (60% amplitude) The reaction

time differed depending on the precursor For the vanadium and

the titania, the sonication time was 10 min The tungsten precursor

required between 5 and 7 min to fully react The sonication was

conducted without cooling so that a temperature of 363 K was

reached at the end of the reaction The resulting suspensions were

centrifuged and the precipitate was thoroughly washed three

times with ethanol (1  20 mL) and THF (2  20 mL) The collected

material was left to dry in air and finally ground into a powder

After grinding, the powders were weighed and good yields (85–

95%) were obtained for all three reactions Care should be taken

as the reaction is rather violent

3 Analysis and characterization

The XRD crystallinity and particle size were investigated by

X-ray diffraction (XRD) XRD measurements carried out using a

Mod-el-2028 (Rigaku) diffractometer Transmission electron microscopy

(TEM) samples were prepared by depositing one drop of the

solu-tion on a 400 mesh copper grid coated with carbon and dried

over-night under vacuum The transmission electron micrographs were

obtained with a JEOL JEM-1200EX electron microscope A Scion

Im-age software program was used to measure the mean particle size

of the nanoparticles from the HRSEM High resolution scanning electron microscopy (HR-SEM) micrographs were obtained using

a JEOL-JSM 700F instrument and a LEO Gemini 982 field emission gun SEM (FEG-SEM) The BET surface area measurements were characterized by nitrogen adsorption at 77 K The domestic micro-wave oven (DMO) operates at 2.45 GHz, under argon atmosphere with a power of 900 W output A white product was obtained for the titanium reaction The tungsten product was yellow The color

of the as-prepared vanadium material was black After heating to

450 °C, the color of the powder turned to orange

4 Results and discussion

In contrast to most of the sol–gel processes that lead to amor-phous materials, our synthesized products are highly crystalline Powder X-ray diffraction (PXRD) patterns of the obtained materials are shown inFig 1 The white titania sample consists of a pure ana-tase phase without any indication of other crystalline products (Fig 1a) The broad diffraction peaks obtained for the titania and the WO3indicate clearly the nanosized nature of the as-prepared products The XRD powder patterns of the TiO2, V2O5, and WO3 show diffraction peaks that match well with the PDF tables (01-086-1157), (03-065-013), and (00-041-0905), respectively The pattern of the yellow particles fits the WO3 data (Fig 1b) The XRD diffraction pattern of the as-prepared black vanadium oxide powder can not be assigned to a specific vanadium oxide phase

We interpret the as-prepared product as being composed of a mix-ture of vanadium oxides EDX measurements of the as-prepared product indicate the presence of only vanadium, oxygen, and car-bon Heating the vanadium mixture to 450 °C has led to the forma-tion of a single-phase crystalline V2O5 (Fig 1c) The average crystallite size estimated using the Scherrer equation is about 10–15 nm for the titania The vanadium and tungsten oxide sizes were about 45–64 nm, and 13–20 nm, respectively (Table 1) It is worth noting that in a recent paper by Niederberger et al.[35], the reaction between VOCl3 and benzyl alcohol has led to the formation of VO1.52(OH)0.77 The reaction was conducted under conventional heating for 24 h at 150 °C That means that conven-tional heating produces different product from our V2O5

Representative high resolution scanning electron microscopy (HRSEM) images are presented inFig 2and were used to calculate the size distribution of the products A highly aggregated morphol-ogy is illustrated inFig 2a, which depicts the titania product The aggregate consists of ‘‘quasi” zero-dimensional particles having a diameter ranging from 3 to 7 nm (Fig 2a) After annealing the vanadium, the oxide sample exhibits rather uniform ellipsoidal particle morphology, ‘‘quasi” one-dimensional, with lengths

rang-Scheme 1 General reaction scheme displaying the metal oxide precursors used the

solvent, the experimental condition and the resulting metal oxide nanoparticles.

2θ (degree)

(a)

(b)

(c)

ing from 150 to 200 nm and diameters varying between 40 and

60 nm (Fig 2b) The length for the mixed valent vanadium oxide,

the as-prepared material, is between 150 and 250 nm, and the

diameter varies between 30 and 60 nm This means that upon

the annealing process, the product does not undergo any

morpho-logical changes

The tungsten oxide forms nearly square, ‘‘quasi”

two-dimen-sional platelets These nanoparticles have facets ranging from 30

to 50 nm (Fig 2c) Side views of particles that are oriented

verti-cally to the SEM grid reveal that the thickness is between 2 and

7 nm These HRSEM images prove that all the three nanomaterials

have a high crystallinity and their size is in good agreement with

the size calculated from the powder XRD patterns recorded for

the same sample

In our case, the BET surface area of the nanoparticles

synthe-sized by microwaves is higher than that synthesynthe-sized by an

ultra-sonic route, apart from titania The BET surface area of TiO2,

V2O5, and WO3, which were prepared in ultrasonic irradiation,

are 162.6, and 47 m2/g, while that of microwave are 124, 12, and

77 m2/g, respectively

Like the surface area, the particle size also depends on the

reac-tion device/method Although particle-size determinareac-tion by peak

broadening is not a very accurate method, the results coincide well

with HRSEM measurements (Table 1) As expected, the decrease in

particle size is expressed in larger surface areas We have also

syn-thesized the same products in a domestic microwave oven The

precursors and the solvents were identical to those in the

sono-chemistry reaction The microwave and sonochemical results of

the BET surface area and the crystallite size of all the materials,

as obtained from HRSEM and from XRD by the Scherrer equation,

are presented inTable 1

4.1 Reaction mechanisms of metal oxide particles in non-aqueous

synthesis

This section will discuss the organic side of the synthesis of

inorganic nanomaterials performed in non-aqueous, but liquid

reaction media The organic components and the organic reaction

pathways play a fundamental role in the non-aqueous synthesis

of the inorganic products In this process, the formal oxygen that

is required for the fabrication of the metal oxide is not provided

by any added water Therefore, it must stem from the precursor

of the organic media In our case, the benzyl alcohol is used as

the oxygen source It is clear that in our process the organic solvent

serves as a solvent as well as a reactant, thus playing a major role

in the formation of the product Generally, two possible organic

mechanisms are suggested for providing the oxygen to enable

the formation of the metal oxide nanostructure The first is the

al-kyl/halide elimination mechanism[26,27] Metal halides are

popu-lar precursors due to their good commercial availability and their

comparatively low cost In alcohol solvents, the alcohol oxygen is

rapidly coordinated to the metal centre, which is followed by an

elimination reaction Basically, two elimination mechanisms can occur: a reaction that directly promotes the formation of the metal oxide by an alkyl halide elimination (Scheme 2, Eq 1), which may

be adequately termed as a hydroxylation process[28] In this case,

a metal-coordinated hydroxyl group is formed that instantly reacts with the precursor to form metal–oxygen–metal groups (Scheme

2, Eq 2) The combination of these two equations leads to the elim-ination of R–X and H–X The other possibility is the elimelim-ination of only hydrogen halide This mechanism constitutes a ligand ex-change reaction[29](Scheme 3, Eq 3)

The second possible mechanism is related to radicals involved

in the reaction as a result of the bubble’s collapse Application of ultrasound to chemical processes involves the use of acoustic cav-itation Acoustic cavitation involves the nucleation, growth, and sudden collapse of the gas of vapor-filled microbubbles formed from acoustical wave-induced compression/rarefaction in a body

of liquid[30] The implosion of the microscopic bubbles in the li-quid generates energy, which induces chemical and mechanical ef-fects It is well known that the sudden collapse leads to localization, a transient high temperature (P5000 K) and pressures (P1000 atm), resulting in an oxidative environment due to the generation of highly reactive species, including hydroxyl radical (

OH)[31,32] This mechanism governs mostly sonochemical reac-tions conducted in aqueous solureac-tions We have examined whether

OH radicals are formed when a bubble collapses in a benzyl alcohol solution The following experimental technique was employed in this probe

ESR: to 20 ml of benzyl alcohol a small amount of a TEMPO trap was added and the sonication took place under identical conditions

to those of the synthesis of the metal oxides It is assumed that if hydroxyl radicals are formed they will be trapped by the TEMPO molecules, and their presence can be detected by electron spin res-onance measurements

Since the possibility of the involvement of hydroxyl radicals is eliminated, we are back to the proposed mechanism of Schemes

2 and 3 To substantiate these mechanisms further experimenta-tion and calculaexperimenta-tion were performed They are outlined below 1

H NMR: the information about the reaction pathway can be found by identifying the organic products, which are present in the final reaction mixture The1H NMR spectra of the pure benzyl alcohol and of the final reaction mixture are presented inFig 3a and b, respectively According toFig 3a of the benzyl alcohol, there are three main peaks The multiplet peaks at 7.25 ppm resulted from the benzene ring proton in the benzyl alcohol structure Addi-tionally, the strong singlet peak at 4.5 ppm refers to the methyl group, whereas the weak singlet peak at 3.3 ppm refers to the

OH group The integration ratios of the benzene ring, the methyl group and the OH peak are 1:0.4:0.2, respectively

The triplet peak at 1.1 ppm and the quartet peak at 3.5 ppm are assigned to the CH3 and CH2 groups of ethanol, respectively According to these benzyl alcohol spectra and the literature values, the1H NMR results show that in a pure TiCl4/benzyl alcohol system,

Table 1

Overview of the surface area and particle sizes calculated from HRSEM and XRD data with a reaction yield.

HRSEM particle size (nm) BET surface area (m 2

/g) Reaction yield (%)

Microwave 45–55 b

Microwave 10–15 b

a

The average crystallite size was calculated using the Scherrer equation.

b

These results are assigned to the average size of the thickness.

the final reaction solution contained benzyl chloride The results/

spectra of the final reaction solution are presented inFig 3b As

op-posed to benzyl alcohol spectra, in the final reaction the spectra

ex-hibit only two peaks The multiplet peaks at 7.33 ppm resulted from the benzene ring proton in the benzyl chloride structure, and the strong singlet peak at 4.66 ppm refers to the methyl chloride group

Scheme 2.

Scheme 3.

Fig 2 HR-SEM micrographs of: (a) crystalline anatase TiO 2 ; mean particle size is 7 ± 1.0 nm, (b) vanadium mixture after heating (single-phase V 2 O 5 ), mean lengths size is

175 ± 20 nm, and mean diameters is 50 ± 10, (c) WO 3 , mean particle size is 41.2 ± 5.9 nm and mean thickness is 6.3 ± 1.5.

This NMR result demonstrates that in this reaction system benzyl

chloride was present as a reaction product

It is well known that sonochemistry is carried out at a much

higher temperature than that of the solution temperature The

endothermic reaction will strongly benefit from this high

temper-ature[33] It was therefore necessary to observe whether the

cur-rent anionic solvothermal reaction is endothermic, and thus

thermodynamic calculations were carried out

DH0

f: the enthalpy of the formation of the synthesis of titanium

oxide from benzyl alcohol and titanium tetra chloride at 298 K was

calculated by using the literature values ofDH0f of the reactants and

the products[34] The enthalpy energy values of TiCl4, benzyl

alco-hol, TiO2, HCl, and benzyl chloride are (804.16), (154.9),

(938.72), (92.3), and (32.6) kJ mol1, respectively

The balanced equation of synthesis titania is the following

reaction:

TiCl4þ 2C6H5CH2—OH ! TiO2þ 2C6H5CH2—Cl þ 2HCl

The calculated value wasDH0f :r= 74 kJ mol1 This value

sur-prised us because it indicates that at 298 K and at 1 atmosphere

of the components, the reaction is exothermic We have also

exam-ined whether the tendency ofDH0f :ris to become more negative or

positive at higher temperatures Our conclusion is (not all the

ther-modynamic values are available) that theDH0

f :rwill become posi-tive at higher temperatures We therefore conclude that the

sonochemistry indeed pushes the reaction towards the products

It is needless to point out that at these high temperatures the

kinetics is indeed much faster Finally, the reaction mechanism of metal oxide particles in non-aqueous systems is proceeding via al-kyl/halide elimination, which is proposed inSchemes 2 and 3, and contains benzyl chloride as a product

5 Conclusion

In summary, transition metal oxide nanoparticles have been successfully prepared using benzyl alcohol–metal chlorides as precursors by a one-step sonochemical method that leads to low-dimensional particle shapes, such as nearly spherical titania nanoparticles, vanadium oxide nanorods, and tungsten oxide nanoplatelets On the one hand, the application of ultrasonic irra-diation offers a fast synthesis route with a low temperature to a variety of metal oxides with high crystallinity, while on the other hand it proposes a non-aqueous, simple method which enables many reaction parameters that are difficult to control in aqueous systems In comparison, in previous reports without ultrasound, the non-aqueous synthesis methods of metal oxide need high tem-peratures or long-time treatments The fact that nanoparticle for-mation is based on the same mechanism in the previous reported and in our report, which done with ultrasound or with the microwave, are strongly supports the proposition that ultra-sound/microwave irradiation has a great potential to control the growth of inorganic nanoparticles through influencing the organic reaction pathway

Scale: 0.4094 ppm/cm, 81.92 Hz/cm

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

Scale: 0.1514 ppm/cm, 30.29 Hz/cm

a

b

Fig 3 (a) 1

H NMR spectra of pure benzyl alcohol (b) 1

H NMR spectra of the final reaction solution (benzyl chloride).

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