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Available online at www.sciencedirect.com

Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 219–225

Size-controlled synthesis and characterization ofquantum-size SnO2 nanocrystallites

by a solvothermal route

Yang Liu a,b, Fan Yang a, Xiurong Yang a,∗a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Renmin Street 5625, Changchun, Jilin 130022, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

Received 15 April 2007; received in revised form 21 June 2007; accepted 28 June 2007Available online 30 June 2007

bstract

By using ethylenediamine as both an alkali and ligand, quantum size SnO2 nanocrystallites were synthesized with a solvothermal route. Theransmission electron micrographs (TEM) were employed to characterize the morphologies of the products. The crystal sizes of the as-synthesizednO2 were ranged form 2.5 to 3.6 nm. The crystal structure and optical properties of the products were investigated by X-ray diffraction, Fourier

ransform infrared spectroscopy, optical absorption spectra, photoluminescence and Raman spectra. Anisotropic growth of the SnO2 nanocrystallitesas observed by altering the solvent from water to ethanol. The SnO2 nanocrystal showed apparent quantum confinement effects. Finally, theechanism for the formation of quantum size SnO2 was also discussed. 2007 Elsevier B.V. All rights reserved.

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eywords: Quantum size; SnO2; Solvothermal; Ethylenediamine

. Introduction

Tin oxide (SnO2) is an important n-type metallic oxideemiconductor with wide band gap (3.6 eV). Because of itsnique electronic, optical, electrochemical and catalytic prop-rties, SnO2 were extensively used in solar cells, transparentonducting electrodes, solid-state sensors, rechargeable Li bat-eries and optical electronic devises [1–6]. The conductivity andptical properties of SnO2 are largely dependent on the par-icle size and shape of the nanocrystallites [7–9]. Along withhe development of nanotechnology, small size nanomaterialsttracted much attention due to their high surface-to-volumeatio, enhanced characteristics of quantum size effects and highraction of chemically similar surface sites [10]. Especially, as

he diameter of SnO2 is smaller or comparable to its excite Bohradius (2.7 nm) that excitons and carriers are confined in allhree dimensions to a nanometer size region, unique properties

∗ Corresponding author. Tel.: +86 431 5262056; fax: +86 431 5689711.E-mail address: xryang@ciac.jl.cn (X. Yang).

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uch as blue shift of the band edge transition energy, unusualtructural and optical properties take place [11,12]. Therefore,broad range of applications in optoelectronics, sensing, laser

nd catalysis are expected.Till now, many methods have been developed to synthe-

ize SnO2 nanocrystallites, including sol–gel [13], chemicalapor deposition (CVD) [14], spray pyrolysis [15], hydrother-al and solvothermal methods [10], laser ablation [16,17], rapid

xidation of metal tin [18], thermal evaporation of oxide pow-ers [19] and molten salt method [20]. To obtain quantum sizenO2 nanocrystallites, the sol–gel method [21] and hydrolysisf SnCl2·2H2O were carried out [10]. Recently, SnO2 quantumots were also fabricated using hydrazine hydrate as the mineral-zer instead of NaOH by a hydrothermal route [15]. However, it istill a great challenge to fabricate the nanostructure SnO2 withontrolled-size and tunable shapes by wet chemical methods2,7,22].

Ethylenediamine was usually used as the structure directiongents for the synthesis of 1D nanomaterials in the solvothermalrocess due to its strong chelating ability with metal ions, andumerous semiconductor with various of morphologies were

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b(A) sample 1, (B) sample 2 and (C) sample 3. The obvious ringsand dots of selective area electron diffraction (SAED) in theinset of Fig. 2 indicate that all the products are polycrystalline.The products of sample 1 synthesized in water (Fig. 2A) are

20 Y. Liu et al. / Colloids and Surfaces A: Ph

ynthesized by using ethylenediamine as solvent [23–25]. In therocess, ethylenediamine acted as a template molecule whichas incorporated into the inorganic framework first and then

scaped from it to form nanocrystallites with desired morpholo-ies [23].

In this work, quantum-size SnO2 nanocrystallites were syn-hesized using ethylenediamine as both an alkali and ligand tooordinate with Sn ions by a solvothermal route. The morpho-ogical and optical properties of the as-synthesis SnO2 werenvestigated by transmission electron micrographs (TEM), X-ay diffraction (XRD), UV-vis spectra, photoluminescence (PL),ourier transform infrared spectroscopy (FT-IT) and Ramanpectra. Furthermore, the possible mechanism for the formationf the SnO2 nanocrystallites was discussed.

. Experimental

.1. Chemicals

All the regents and solvents used were analytical grade (Bei-ing Chemical Reagent Co.) and were used without any furtherurification.

.2. The preparation of SnO2 nanocrystal

0.53 g of SnCl4·5H2O were taken into a beaker with 30 mLf solutions (sample 1, deionized water; sample 2, a mixturef water and ethanol in equal volume ratio; sample 3, ethanol).ill the salts were dissolved absolutely, 0.4 mL of ethylenedi-mine was slowly dropped into the beakers. Slurry-like whiterecipitations were formed. After stirring for 5 min, the solutionas transferred into Teflon-line stainless steel autoclave with a

apacity of 40 mL. The autoclaves were stand at 180 ◦C for 24 hnd were tap-cooled. The final products were centrifuged andashed with ethanol and deionized water for several times and

hen dried at 60 ◦C in vacuum. White powders were obtained.

.3. Characterization

X-ray powder diffraction measurements were performed on a/max 2500V PC X-ray diffractometer using Cu-K�-radiation

1.5406 A) of 40 kV and 30 mA.Transmission electron micrographs and the selected area

lectron diffraction (SAED) were taken with a JEOL-JEM-2010perating at 200 kV (JEOL, Japan).

XPS was conducted using a VG ESCALAB MK II spectrom-ter (VG Scientific, UK) employing a monochromatic MgK�-ray source (hυ = 1253.6 eV). Peak position was internally

eferenced to the C1s peak at 284.6 eV.FT-IR was conducted using a FTS135 infrared spectroscopy

BIO-RAD, USA).UV-vis spectrum was acquired using a Cary 50 UV-vis NTR

pectrometer (Varian USA).Room-temperature photoluminescence measurement was

arried out on a Pekin-Elmer LS 50B spectrometer by excitinghe sample with a He–Cd laser at 310 nm.

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hem. Eng. Aspects 312 (2008) 219–225

. Results and discussion

X-ray powder diffraction was usually used to determinatehe crystal structure, phase and purity of the samples. Fig. 1Ahows the XRD patterns of (a) sample 1, (b) sample 2 and (c)ample 3. All samples present wide diffraction peaks at the sameosition, which can be indexed to the tetragonal rutile structuref SnO2 (JCPDS card no. 41-1445) with lattice constants of= 4.737 A and c = 3.186 A. Moreover, no other characteristiceaks of the impurities, such as metallic Sn and other tin oxidesere observed. The facts indicate that the level of impurity in

he samples is low. The broad peaks are attributed to their smallizes according to the Scherer’s equation:

h k l = 0.89λ

(β cos θ)

here the λ is X-ray wavelength (0.15418 nm for Cu-K�), β ishe width at half maximum of the diffraction peak (fwhm) and

is Bragg diffraction angle [26]. The mean grain sizes of as-ynthesized SnO2 nanocrystallites of sample 1, sample 2 andample 3 were calculated to be 3.6, 3.4 and 2.6 nm, respec-ively. Comparing the relative intensity of the diffraction peaks,t can be seen that the relative intensity rate of (1 0 1) to (1 1 0)f the diffraction peaks of sample 1 is 0.75, which is consis-ent with the value from the standard card (JCPDS card no.1-1445), but it increases to 0.87 of sample 2 and 1.1 for sample. This phenomenon was also observed in the synthesis of SnO2anorods etc. by hydrothermal route, which was ascribed to thenisotropic growth of SnO2 nanocrystallites in the solvothermalrocess [22].

The morphologies and particle sizes were further investigatedy TEM. Fig. 2 shows the TEM images of the SnO2 samples of

ig. 1. The XRD patterns of the products prepared in water (a), the mixture ofater and ethanol (b), ethanol (c) for 24 h and in ethanol for 12 h (d).

Y. Liu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 312 (2008) 219–225 221

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ig. 2. The TEM pictures (A, B, C) and diameter size distribution histograms (nd ethanol (B, E), ethanol (C, F) for 24 h, respectively.

omogeneous ultrafine nanoparticals, and the diameters of the

anoparticles were ca. 3.6 nm. The size of the nanocrystallites isomparable to the Bohr radius (2.7 nm) of SnO2 and is similaro that calculated from Scherer’s equation. Fig. 2B reveals TEMattern of sample 2 prepared in the mixture of water and ethanol.

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F) of the SnO2 nanocrystallites prepared in water (A, D), the mixture of water

omparing to that of sample 1, slightly elongate nanoparticles of

nO2 nanocrystallites were obtained and the size of the productsas decreased to ca. 3.3 nm. Moreover, the particles were aggre-ated. The presence of the elongate crystals indicates that therowth of the SnO2 crystal is followed the grain rotation induced

2 ysicochem. Eng. Aspects 312 (2008) 219–225

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rain coalescence mechanism [7,27]. Fig. 2C reveals the mor-hology of SnO2 nanocrystallites prepared in ethanol. As it cane seen that the samples are well dispersed and are quasi-1Dhort rod-like nanoparticles with length of ca. 5.6 nm and widthf ca. 2.5 nm besides few nanoparticles with size of 2.5 nm arebserved. Comparing the TEM patterns, we can clearly observehe anisotropic growth trend of the SnO2 with the increasingccount of ethanol in the precursor, which are well consistentith those of XRD. Moreover, the particle sizes of all the samplesbtained from TEM patterns are quite similar to those calculatedorm Scherer’s equation.

To further obtain the composition and chemical states of theroducts of SnO2, the as-synthesized products were investigatedy XPS. Fig. 3 reveals the XPS patterns of (a) sample 1, (b)ample 2 and (c) sample 3. For all the samples, the same peakosition of the 3d spin-orbit of Sn is obtained. The peaks at86.9 and 495.3 eV are respectively corresponding to the spin-rbit of 3d5/2 and 3d3/2 and the area ratios of the peaks are ca..5, which indicate that the chemical states of all the products aren4+ [28]. On the other hand, the O1s peaks (not shown) of all as-repared products present one peak at 530.9 eV, correspondingo crystalline oxygen of the Sn O Sn bond. The XPS resultsonfirm that the compositions of all the products are SnO2. Theelative atomic concentration of Sn and O were also analyzedased on XPS spectra. The atomic mole ratios of O to Sn are 1.95,.96, 1.98 for sample 1, sample 2 and sample 3, respectively,nd are nearly the same as the standard stoichiometric ratio ofnO2.

The small size effect and structural information were alsonvestigated by Raman spectra. Fig. 4 shows the Raman spectraf the SnO2 nanocrystallites. The tetragonal rutile SnO2 belongso the space group D14

4h with two SnO2 molecular per unit cell.he normal lattice vibration at the Γ point of the brillouin zone

s Γ = 1A1g + 1A2g + 1A2u + 1B1g + 1B2g + 2B1u + 1Eg + 3Eu for

he 11 optical phonons of symmetry [29]. In the irreducible rep-esentations of the optical modes, A1g, B1g, B2g and Eg areaman active. The intensity of mode A1g at 631 cm−1 is the

trongest, followed by the mode B2g at 774 cm−1. The other

ig. 3. The XPS results of Sn element for the products prepared in water (a),he mixture of water and ethanol (b), ethanol (c) for 24 h.

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ig. 4. The Raman spectra of the SnO2 nanocrystallites prepared in water (a),he mixture of water and ethanol (b), ethanol (c) for 24 h.

odes are very weak. For the sample 3 as shown in Fig. 4a,he peaks at 631 and 771 cm−1 can be assigned to A1g and B2g,espectively, which are relative to the expansion and contrac-ion vibration modes of Sn O bonds in the rutile structure ofnO2. The Raman bond at 476 cm−1 is ascribed to B2g mode.he main photon modes are also observed in the Raman spectraf sample 1 and sample 2. Except for the main characteristiceaks of bulk SnO2 powder, other three peaks at 353, 434,71 and 882 cm−1 are also observed in the Raman spectra ofs-synthesized SnO2. The Raman bonds at 353 and 571 cm−1

ere usually reported in extra-fine nanoparticles [30–32] andanorods [6,33] and were related to the small size effect accord-ng to the Matossi force constant model [34], and it was noteported in the larger SnO2 crystal. The Raman bonds at 353nd 571 cm−1 are also observed for the sample 1 and sample. The facts further confirm the characteristics of the tetrago-al rutile structure of as-synthesized SnO2 products as well ashe small size of the SnO2 nanocrystal. On the other hand, theaman bonds at 434 and 882 cm−1 can be resulted from the

udimental NH2 group [35].Fig. 5 shows the FT-IR spectra of the as-synthesized prod-

cts (a) sample 1, (b) sample 2 and (c) sample 3. For all theamples, the peaks at 3419, 1641 and 1211 cm−1 exist, whichre attributed to the adsorbed water on the samples of SnO2anocrystal [36]. The wide band at 3419 cm−1 is mainly thetretching vibration mode of O H group on the surface of theroducts and the peak at 1641 cm−1 is the O H bending band,hich is associated with the adsorbed water on the products.or the samples 3 (Fig. 5c), two peaks at 559 and 673 cm−1 arebserved apparently. The bond at 559 cm−1 is attributed to theibration of the Sn OH terminal bonds and the band at 673 cm−1

s associated with the anti-symmetric O Sn O stretching modef the surface-bridging oxide formed by condensation of adja-

ent surface hydroxyl groups [7,37]. For the FT-IR of sample 1nd sample 2, the two peaks at 500–700 cm−1 cannot be seennstead of the existence of a wide peak, which can be owing tohe overlap of the two peaks.

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nThe mechanism for the formation of the SnO nanocrystallites

ig. 5. The FT-IR spectra of the products prepared in water (a), the mixture ofater and ethanol (b), ethanol (c) for 24 h and in ethanol for 12 h (d).

Fig. 6 presents the optical absorbance spectra of the SnO2anocrystalline for (a) sample 1, (b) sample 2 and (c) sample. As it is stated above, when the size of SnO2 nanocrystallitess smaller or comparable to the exciton Bohr radius, the quan-um confinement effect would occur and a blue shift in energys observed. The gap band is found to be particle size depen-ent and increases with decreasing particle size. The absorptionpectrum edge will shift to higher energy concomitantly. Thedsorption onset of the sample 1, sample 2 and sample 3 are12, 323 and 337 nm. The absorption edges are red shift uponarticles growth, indicating that the sizes of as-synthesis SnO2anocrystallites are in the quantum regime. The bond gap energyEg) for the SnO2 nanocrystallites can be obtained by extrapo-ation of the rising part to the plot to the x-axis as shown in

ig. 6. The corresponding band gap energy can be calculated toe 3.97, 3.83 and 3.68 eV and is larger than the value of 3.6 eVor the bulk SnO2 [38]. The increasing trends of the band gap

ig. 6. The UV-vis spectra of the SnO2 nanocrystallites prepared in water (a),he mixture of water and ethanol (b), ethanol (c) for 24 h.

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nergy upon the decreasing particle size are well presented forhe quantum confinement effect.

Fig. 7 shows the room temperature PL of the as-preparednO2 nanocrystallites of sample 1, (b) sample 2 and (c) sam-le 3 with an excitation wavelength of 310 nm. In general, thexygen vacancies are the most common defects in the poly-r nanocrystalline oxide and are used to be the radiative cen-er in the luminescence process. The oxygen vacancies mostlyresent in three charge states of VO

0, VO+ and VO

++ [7,41].s the VO

0 is a much shallow donor, it is believed that mostxygen vacancies will be in their paramagnetic VO

+ state underat-band conditions [41]. In the long wavelength, all the sam-les show a PL peak centered at 600 nm which is usually seenn the samples of SnO2 nanoribbons, nanorods and nanowires,32,33,39,40] which would be related to the crystalline defectsuring the growth process. It is worth noting that there are threeeaks in the visible emission of 400–500 nm for all the SnO2roducts. The peaks might also come from the luminescenceenter of tin interstitials or dangling etc. in the present SnO2anoparticles [37]. The earlier reports supposed that the broadeak around 400–500 nm can be assumed to be due to the for-ation of a VO

++ luminescent center in the SnO2 nanocrystalnd nanorod [2,3,7,33]. The recombination of surface trappedole with an electron in deep trap (VO

+) to form a VO++ center

ives rise to visible emission when a conduction band electronecombines with the VO

++ center. Moreover, the center peak ofample 3 (440 nm, Fig. 7c) in the region of 400–500 nm shows aittle red shift of 5 nm, comparing to the peak at 435 nm of sam-le 1 and sample 2, which is supposed to be resulted from theesidual stresses within the SnO2 nanocrystallites that originaterom the lattice distortion [39].

It is well known that ethylenediamine is both a strong coordi-ation that can coordinate with Sn4+ and a strong alkali reagent.

2ould be meaningful to provide the methodology to synthe-

is novel nanomaterials. To investigate the mechanism for theormation of the SnO2 nanocrystallites, the effects of solvother-

ig. 7. The room temperature PL of the SnO2 nanocrystallites prepared in watera), the mixture of water and ethanol (b), ethanol (c) for 24 h.

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al time upon the structure and composite of the products weretudied. Fig. 1d shows the XRD pattern of sample 4 which is syn-hesized similarly to that of sample 3 except that the solvothermalime is decreased to 12 h. The XRD results of sample 4 is quiteifferent from that of other samples and many new peaks appearxcept for the main peaks of SnO2 crystal, which apparentlyriginate from the complex between Sn4+ and ethylenediamine,nstead of SnO2. On the other hand, the FT-IR results werelso investigated as shown in Fig. 5d. Besides the main peaksescribed before, many other peaks were observed. In the highrequency region, the band at 3192 cm−1 is attributed to the N Htretching vibration. The strong bands of 2913 and 2800 cm−1

re ascribed to the asymmetric and symmetric stretching modesf CH in the hydrocarbon chain of ethylenediamine [38]. Theharp peaks at 1600 and 1510 cm−1 are assigned to the defor-ation modes of N H bond in the internal face, and the peak at

20 cm−1 is attributed to the deformation mode of N H bondn the external face. It is obviously that both νNH and δNHodes shift toward lower frequency comparing with those of

ree NH bond, which can be attributed to the hydrogen bond42]. The numerous of peaks in the region of 2500–3000 cm−1

an also be resulted from the formation of chelating compound.n addition, the peaks at 1020–1220 cm−1 are ascribed to C Ntretching vibration modes. These facts indicated that the com-lex compounds formed in the early stage. The strong peakt 2056 cm−1 appears which can originate from cyanogens oriamineacetylene species that comes from the decompositionf ethylenediamine at high temperature [43]. From the aboveesults, the possible mechanism for formation of quantum-sizenO2 nanocrystallites during the solvothermal process was sup-osed as followed: as the ethylenediamine was added into therecursor solutions, the complex of Snn(ethylenediamine)m

4+

ere formed immediately and white slurry was observed. Mean-hile, OH groups were released. During the solvothermalrocess, the complex of Snn(ethylenediamine)m−x(OH)x

(4−x)+

ere dissociated and SnO2 nanocrystallites formed gradually.t was thought that the formation of Snn(ethylenediamine)m

4+

omplex decreased the reactivity between Sn ion and OH groupnd resulted in the formation of quantum size SnO2 nanocrys-allites.

. Conclusion

In summery, quantum size SnO2 nanocrystallites wereynthesized successfully by solvothermal route using ethylene-iamine as both a coordination regent and an alkali reagent. Thearticles sizes of the SnO2 nanocrystallites prepared in water,ixture of water and ethanol (v/v = 1:1) and ethanol were 3.6,

.4 and 2.5 nm, respectively, which is conformed by both TEMnd XRD results. On the other hand, the morphologies of the as-ynthesis SnO2 nanocrystallites are dependent on the solvent.

oreover, obvious quantum size effect on the SnO2 nanocrys-allites was observed from the UV-vis spectra, Raman spectra.

t was supposed that the formation of complexes between Sn4+

nd ethylendiamine decreased the releasing rate of Sn4+ andnduced the formation of quantum size SnO2. This work woulde meaningful to provide a methodology to synthesize ultrafine

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hem. Eng. Aspects 312 (2008) 219–225

anomaterials. Moreover, the quantum size SnO2 nanocrystal-ites would be promised in the applications of sensor, solar cellnd optical electronic devises.

cknowledgements

This work was supported by the National Key Basic Researchevelopment Project Research on Human Major Disease Pro-

eomics (no. 2001CB5102) and the National Nature Scienceoundation of China with the grant (no. 20475052).

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