Journal of Crystal Growth 314 (2011) 171–179

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

n Corr

Chonbu

South K

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Structural evolution of SnO2 nanostructure from core–shell faceted pyramidsto nanorods and its gas-sensing properties

Soumen Das a, Dae-Young Kim a, Cheol-Min Choi a, Y.B. Hahn a,b,n

a School of Semiconductor and Chemical Engineering, Chonbuk National University, 664-14 Duckjin-Dong 1 Ga, Jeonju 561-756, South Koreab WCU Department of BIN Fusion Technology, Chonbuk National University, 664-14 Duckjin-Dong 1 Ga, Jeonju 561-756, South Korea

a r t i c l e i n f o

Article history:

Received 14 July 2010

Received in revised form

28 September 2010

Accepted 22 October 2010

Communicated by J.M. Redwingdependent on the molar concentration of HMTA (1–10 mM) in aqueous solution. It was revealed that the

Available online 7 November 2010

Keywords:

A1. SnO2 nanostructures

A1. Core–shell type pyramids

A1. Nanorods

B2. Semiconducting materials

B3. Gas-sensing properties

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.jcrysgro.2010.10.151

esponding author at: School of Chemical E

k National University, 664-14 Duckjin-Dong 1

orea. Tel.: +82 63 270 2439; fax: +82 63 270

ail address: ybhahn@chonbuk.ac.kr (Y.B. Hahn

a b s t r a c t

Tin oxide (SnO2) nanorods were synthesized through an aqueous hexamethylenetetramine (HMTA)

assisted synthesis route and their structural evolution from core–shell type faceted pyramidal assembly

was investigated. Structural analysis revealed that the as-synthesized faceted SnO2 structures were made

of randomly arranged nanocrystals with diameter of 2–5 nm. The shell thickness (0–80 nm) was

self-assembly was possible only with tin (II) chloride solution as precursor and not with tin (IV) chloride

solution. At longer synthesis hours, the pyramidal nanostructures were gradually disintegrated into

single crystalline nanorods with diameter of about 5–10 nm and length of about 100–200 nm. The SnO2

nanorods showed high sensitivity towards acetone, but they were relatively less sensitive to methane,

butane, sulfur dioxide, carbon monoxide and carbon dioxide. Possible mechanisms for the growth and

sensing properties of the nanostructures were discussed.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Rutile tin oxide (SnO2) is a stable n-type wide band gap (3.57 eV)semiconductor. Due to the good quality electrical, optical andelectrochemical properties, it offers wide range of applications insolar cells [1–3], high-capacity lithium-storage and catalytic sup-port materials [4–8]. Moreover, SnO2 has high carrier density andsupports enormous concentration of intrinsic and stoichiometryviolating vacancies. These vacancies are correlated with its elec-trical conductivity and felicitate the maximum use as gas-sensingmaterial [9–11]. Many forms of nanostructural SnO2 have beensynthesized by various methods and their gas-sensing propertieshave been studied [9,11–16]. In course, many attempts have beenmade to grow variety of nanostructures by employing suitablepolymer chains as molecular template or to facilitate desired self-assembly [17–21]. Though regarded as low-cost as well as con-venient, yet it is still a great challenge to obtain nanostructureswith tunable dimension by wet chemical synthesis technique[16,22,23]. Thus, in oil-in-water microemulsion process, the struc-tural modifications are achieved by an interfacial process driven bythe hydrophobic van der Waals interaction. Otherwise, for singletailed surfactants, an alkane chain of eight or more carbon isrequired to stabilize nanocrystal micelles, e.g. gold nanocrystals

ll rights reserved.

ngineering and Technology,

Ga, Jeonju 561-756,

2306.

).

stabilized by C12 alkanethiols (dodecanethiol) [24]. In addition,through a hydrothermal self-assembly process, the nanocrystalmicelles are successfully utilized as building blocks in the synthesisof new, supported and hierarchically ordered inorganic mesophasecrystals [25]. In this case the nanocrystals–micelles provide multi-functional interface for further self-assembly with metal oxidesthrough charge interactions and hydrogen bonding [26,27]. Chenget al. [28] reported synthesis of single crystalline cone shaped andself-assembled SnO2 nanostructures by a polyacrylic acid (or PAA)assisted solvothermal process. It is assumed that the long-chainPAA directs the aggregation of colloidal particles and leads tosubsequent crystallization. Liu et al. [29] achieved nanowire likemorphology by employing polyethylene glycol (PEG) as structure-directing agent. The PEG polymer acts as heterogeneous nucleationsites at the polymer/water interface. Wang et al. [30] reported ashape specific synthesis of one-dimensional soluble nanorods andbipods of SnO2 with small diameters based on the oleyamine-assisted hydrolysis of tin alkoxide in the presence of high content ofoleic acid. Despite these findings summarized above, there arescopes to study the tunable nanostructural formations of self-assembled SnO2 nanocrystals through wet chemical synthesistechnique.

In the present work, we study self-assembly of SnO2 nanocrys-tals through wet chemical synthesis technique. We demonstratedthat aqueous HMTA solution could be used as structure-directingagent to produce single crystalline SnO2 nanorods. A synthesisscheme was thus developed to ascertain the role of HMTA and tin(II) chloride in the structural evolution. Study also revealed that the

S. Das et al. / Journal of Crystal Growth 314 (2011) 171–179172

gas response of these nanorods was highly sensitive towardsacetone and least towards CO2. The mechanism of such activitiesis discussed herein.

2. Experimental procedure

The synthesis process involved is as follows. Hexamethylenete-tramine and tin (II) chloride (SnCl2 �2H2O) were purchased fromSigma-Aldrich (99.9% purity) and were used without furtherpurification. SnCl2 �2H2O was dissolved in 100 mL water withdifferent molar concentrations (see Table 1). In another separatebeaker different molar concentrations of HMTA were dissolved in100 mL water at low temperature (in ice water bath; see Table 1).Both the portions were stirred for 5 min. The aqueous HMTAsolution looks transparent and the aqueous tin chloride solutionwas transparent (1 mM) or translucent (5 mM). After 20 min, theaqueous HMTA solution was slowly mixed with the aqueous SnCl2

solution under constant stirring. Interestingly, in each case, initiallythe mixture solution was transparent, but turned turbid oncearound 60 mL of the aqueous HMTA solution was mixed into it. Inthe present synthesis scheme, the aqueous tin (II) chloride solutionhas a pH of �2.7, whereas aqueous HMTA solution shows pH�7.75. The mixture solution registered the pH of �3.9. Theisoelectric point (PI) or the point of zero charge of SnO2 is �4–5[31]. Therefore, during the mixing of aqueous basic HMTA solution,the pH of the solution changes and the pH corresponds toapproximate PI value at which precipitation starts. The obtainedthick white substance was stirred at room temperature and wasthen transferred into a water bath preheated at 90 1C and wastreated for few minutes to few hours. The powder was collected,washed and dried in vacuum. The as-synthesized powder sampleswere then used for morphological and other characterizations.Table 1 summarizes the details of the sample names, synthesisconditions (i.e., molar concentrations of HMTA and tin precursors,pH and duration of synthesis) and obtained morphologies.

The morphology and crystalline size of the SnO2 sampleswere determined by field emission scanning electron microscope(FESEM, JSM S4800) and by transmission electron microscope(TEM, JTEM 2010). X-ray photoelectron spectra (XPS) of thesamples were recorded with a Thermo K-alpha ESKA System witha monochromatic Al-Ka source and a charge neutralizer.

For the gas-sensing measurements thick pastes of the SnO2

nanorods were prepared in an aqueous medium containing a smallamount of polyvinyl alcohol (PVA) binder. The pastes were paintedon the outer surface of alumina tubes (length 3 mm, outer diameter2 mm and thickness 0.5 mm). The details of sensor fabrication and

Table 1Summary of sample names, molar concentrations of HMTA and tin precursors, pH, syn

Sample SnCl2 �2H2O (mM) HMTA (mM) pH

S1 1 1 3.94

S2 1 2 3.94

S3 1 5 3.94

S4 1 7 3.76

S5 1 10 3.43

S6 1 10 3.43

S7 1 10 3.43

S8 1 10 3.43

S9 5 10 3.91

S10 5 10 3.91

S11 5 10 3.91

S12 5 10 3.91

S13 5 10 3.91

S14 5 15 4.1

S15 5 20 4.2

S16 5 30 4.25

packaging are given elsewhere [32]. Gold electrodes and platinumlead wires were attached at the ends of the tubes (by curing at ahigher temperature) before applying the paste. After painting, thecoated alumina tubes were baked at 600 1C for 1 h. Kanthal heatingcoils were placed inside the tubes and the leads were bonded tonickel pins. The electrical resistance and gas sensitivity of thecoatings were measured at different temperatures (up to 450 1C byplacing a Kanthal heating coil inside the coated alumina tubes) inan ambient relative humidity of 60–70% using a digital multimeter(Solartron), a constant voltage/current source (Keithley 228A) andan X–Y recorder (Yokogawa).

3. Results and discussion

3.1. Structural evolution of SnO2

Fig. 1 shows the FESEM (a)–(e) and TEM images (f)–(j) of thefaceted pyramidal shaped structures obtained in different synth-esis conditions. High-magnification TEM images of the samestructures with signatures of particle assembly resembling outershell around a central thick part are shown in Fig. 1(k)–(o).Interestingly, we have found that the shell thickness varies withthe molar concentration of aqueous HMTA. High-magnificationFESEM images reveal that the planar surfaces of pyramid are madeof numerous smaller nanocrystals (see Fig. 1 of the supportinginformation). Fig. 2 shows the high-resolution microscopic imagesof the shell-like arrangements obtained at different molar con-centrations of HMTA, 0 nm with 1 mM (a), �20 nm with 2 mM (b),�30 nm with 5 mM (c), �40 nm with 7 mM (d) and �80 nm with10 mM (e). S1–S5 in Fig. 2 represents sample numbers (also seeTable 1). Fig. 2(f)–(j) reveal the polycrystalline character of thesamples where the sizes of the consisting nanocrystals weredetermined as around 2–5 nm. The selected area electron diffrac-tion (SAED) ring pattern in Fig. 2(f) and the lattice fringes inFig. 2(g)–(h) correspond to the (1 1 0), (1 0 1), (2 0 0) and (2 1 1)parallel planes of rutile SnO2 with spacing �0.345, 0.267, 0.236 and0.176 nm, respectively [33]. For higher molar concentration ofHMTA (410 mM), irregular and featureless structures wereobtained (see Fig. 2 of the supporting information).

Fig. 3 shows the morphological evolution with the duration ofsynthesis to 2 h (a), 12 h (b) and 24 h (c). We observed that withlonger synthesis time, the surface of the primary structure attachesmore nanocrystals and gradually gets thicker. So, in Fig. 3(a) weobserve dense central part surrounded by wide shell-like arrange-ment. Fig. 3(b) shows an absence of central part and reveals some ofthe thicker edges, which eventually protrude out of the assembly.

thesis time and obtained structural shapes.

Time Relative concentration Shape

5 min 1:1 Pyramid

5 min 1:2 Pyramid

5 min 1:5 Pyramid

5 min 1:7 Pyramid

5 min 1:10 Pyramid

2 h 1:10 Pyramid

12 h 1:10 Pyramid/nanorod

24 h 1:10 Pyramid/nanorod

2 h 1:2 Pyramid/nanorod

4 h 1:2 Pyramid/nanorod

6 h 1:2 Pyramid/nanorod

12 h 1:2 Pyramid/nanorod

24 h 1:2 Nanorod

5 min 1:3 Pyramid

5 min 1:4 Pyramid

5 min 1:6 pyramid

Fig. 1. FESEM images of the morphological variation in the rutile SnO2 samples as a function of relative molar concentration of SnCl2 �2H2O to HMTA: (a) 1:1 (S1), (b) 1:2 (S2),

(c) 1:5 (S3), (d) 1:7 (S4) and (e) 1:10 (S5). The corresponding TEM images are shown in (f) S1, (g) S2, (h) S3, (i) S4 and (j) S5. All structures are faceted pyramidal type. High-

magnification TEM images in (k)–(o) show shell-like arrangement around a thick central core.

S. Das et al. / Journal of Crystal Growth 314 (2011) 171–179 173

Fig. 3(c) reveals growth of nanorod-like assembly. It is evident thatthese nanorods grew from the thicker edges of the shell once weincreased the duration of synthesis (see Fig. 3 of the supporting

information for additional images). In order to realize completeformation of the nanorods, the molar concentration of the pre-cursor tin (II) chloride was increased to 5 mM. Thus, Fig. 4 shows

Fig. 2. High-magnification TEM images of the core–shell like assembly of SnO2

(a–e): the shell thickness varied from (a) �0 nm, (b) �20 nm, (c) �30 nm, (d)

�40 nm and (e) �80 nm. High-resolution TEM images of the nanocrystal with sizes

�2–5 nm (f–i) the polycrystalline core–shell like assembly. The selected area

electron diffraction pattern in (f) and parallel lattice planes in (g–j) correspond to

(1 1 0), (1 0 1), (2 0 0) and (2 1 1) of rutile SnO2.

Fig. 3. TEM images showing the synthesis-time-dependent morphological varia-

tions in rutile SnO2: (a) 2 h, (b) 12 h and (c) 24 h. The images revealed that with

longer duration of synthesis, the outer shell (surface) of the faceted structures

thickens as newer constituent particles attached to it (S7), which eventually breaks

away to form rod like arrangement (S8). Note the absence of thick core like assembly

in S7 (b) and also in S8 (c).

S. Das et al. / Journal of Crystal Growth 314 (2011) 171–179174

the evolution of SnO2 nanorods with the duration of synthesis insuch case. Fig. 4(a)–(e) show that as the synthesis time wasincreased from 2 h (a) to 24 h (e), the pyramid like structuresfinally disintegrated into nanorods with diameter of about5–10 nm and length of about 100–200 nm (Fig. 4(f)–(j)). The fast

Fourier transformation (FFT) images of the nanorods are shown atthe inset of the corresponding images. Thus it is concluded thatwith the long duration of synthesis the outer shell of the facetedstructures gets thicker by continual attachment of the primarynanocrystals, which finally breaks away to form elongated nanor-ods. The continual detachment of the outer shell exposes the innerlayer, which in turn attaches additional nanocrystals. This con-tinuous process eventually converts the entire faceted structure

Fig. 4. TEM images exhibiting the formation of single crystalline nanorods as a function of synthesis time (a) 2 h, S9; (b) 4 h, S10; (c) 6 h, S11; (d) 6 h, S12 and (e) 24 h, S13. Images

revealed that nanorods are structurally derived from the pyramid. Corresponding high-magnification TEM images in (f–j) show the single crystalline nanorods with length of about

100–200 nm and diameter of about 5–10 nm. The high-resolution TEM images (k–o) reveals (1 1 0) parallel lattice planes of the obtained nanorods with spacing �0.339 nm.

S. Das et al. / Journal of Crystal Growth 314 (2011) 171–179 175

into an assembly of single crystalline nanorods. The single crystal-line nature of the nanorods can be observed from the high-resolution TEM images as shown in Fig. 4(k)–(o).

Insets in Fig. 4(a) and (e) show the SAED ring and spots patterns ofthe corresponding samples, respectively. The patterns correspond to(1 1 0), (1 0 1), (2 0 0) and (2 1 1) lattice planes of rutile SnO2 [33].Significantly, the spacing between the lattice planes of the singlecrystalline nanorods perpendicular to the growth direction is�0.339 nm, related to the parallel (1 1 0) planes (Fig. 4(k)–(o)). The

lattice spacing parallel to the growth direction is �0.267 nm related to(1 0 1) parallel planes. In the rutile SnO2 crystal structure the sequenceof the surface energy per crystal plane is (1 1 0)o(1 0 0)o(1 0 1)o(0 0 1) [34]. Due to the lowest surface energy, the plane (1 1 0) is thethermodynamically most stable plane. Moreover, (1 1 0) surface has nonet dipole in the [1 1 0] direction and therefore it is a nonpolar surface.Thus, it is assumed that the slow generation of the nuclei in an ionicatmosphere may induce one-dimensional growth of rutile SnO2 inorder to achieve thermodynamically stable arrangements.

0 200 400 600 800 1000

OK

LL

Sn3sSn

3p1/

2

Sn3p

3/2

O1S

Sn3

d 3/2

Sn3d

5/2

C1S

Sn4s

Sn4p

Sn4dIn

tens

ity (

arb.

uni

t)In

tens

ity (

arb.

uni

t)In

tens

ity (

arb.

uni

t)

Binding energy (eV)

486 488 490 492 494 496 498 500

Sn+4 3d5/2

Sn+4 3d3/2

Binding energy (eV)

S1

S5

S13

528 531 534 537528 531 534 537528 531 534 537

S1

Binding energy (eV)

S5 S13

Fig. 5. Oxidation state of the SnO2 nanostructures from XPS study shows (a) a represen-

tative survey spectrum in the range of 0–1000 eV, (b) Sn3d region, which is characterized

by the spin–orbit splitting of the Sn3d5/2 ground state at �487.6 eV, while the Sn3d3/2

excited state at �496.1 eV is attributed to Sn4+ oxidation state of the S1, S5 and S13 SnO2

and (c) asymmetric O1s main peak at �531.5 eV assigned to the lattice oxygen, while the

shoulder at 532.9 eV is considered to be due to the oxygen of the Sn�OH bonds.

S. Das et al. / Journal of Crystal Growth 314 (2011) 171–179176

The oxidation states of SnO2 samples were determined from XPSstudy. In Fig. 5(a), apart from the weak C1s peak at around�286.5 eV, only Sn- and O-related core levels are detected inthe analysis. The Sn3d region is characterized by the spin–orbitsplitting of the Sn3d5/2 ground state at �487.6 eV, while the Sn3d3/2

excited state at �496.1 eV is attributed to Sn4 + oxidation state ofthe samples (Fig. 5(b)) [35]. The clear symmetric peaks suggest thatthere are no sub-peaks among them. So, we may rule out thepresence of Sn2 + oxidation state for these oxides. The obtained O1speaks were deconvoluted into two components by fitting itaccording to the Lorenztian function. The main peak is identifiedat �531.5 eV, while the shoulder at 532.9 eV (Fig. 5c–e). The mainpeak is assigned to the lattice oxygen and the shoulder is due to theoxygen of the Sn–OH bonds [36].

3.2. Self-assembly growth mechanism of SnO2 nanostructures

The growth of the SnO2 nanostructures can be described as acollective process of self-assembly and oriented attachmentmechanism. Banfield et al. [37] reported that owing to theBrownian motion, the jiggling of nanoparticles may allow adjacentparticles to rotate and then find a low-energy configuration. So, wepresume that in the present case, the nanocrystals first rotate toalign along the [0 0 1] direction and form chainlike structures, itthen grows in order to minimize the surface energy. It is reportedthat tin (II) chloride forms polymeric chain in the aqueous solutioncaused by hydrogen bonding between water molecules [38].As aqueous HMTA is heated, it hydrolyzes and releases OH� inthe solution by the following chemical reactions:

(CH2)6N4+6H2O-6HCHO+4NH3 (1)

NH3+H2O2NH4+ +OH� (2)

SnCl2+2H2O-Sn(OH)2+2HCl� (3)

Sn2 + +4H2O-Sn(OH)4+2e� +4H+ (4)

Sn(OH)4-SnO2+2H2O (5)

The released OH� hydroxyl ion in a way affects the nucleation andgrowth behaviour of the tin oxide nanocrystal [39]. An optimized OH�

concentration ascertains the metal–oxygen–metal bonds in an orderedfashion. Thus the O–Sn–O bridges are formed from structural OHgroups belonging to two neighbouring tin atoms, resulting in (1 1 0),(1 0 1) and (1 0 0) planar structural arrangement. The (1 1 0) surface isformed by alternating layers (O(�2)–Sn2O2(+4)–O(�2)) with zerocharge and the net dipole of the surface is zero. The (1 0 1) surface alltins are five-fold coordinated, and the equatorial oxygen is absent. Theresulting surface is best formulated as (O2(4�)–Sn2(8+)–O2(4�)) andit is also nonpolar [40]. On the other hand at low temperature thepresence of four symmetrically placed N atoms and the overall cubicsymmetry of the HMTA molecule allow the formation of up tofour hydrogen bonding interactions: N–H?O or O–H?N, O–H?O,C–H?O and N–H?N [41]. The formation of N?H bond involvesmainly the unpaired electron of the nitrogen atom. The strong N?Hbond with the one nitrogen causes distortion of the HMTA moleculeand also causes a large electric field gradient for the other three atoms[42]. It has been suggested that due to incompatibility, the nonpolarsurface in close proximity with another polar surface rearranges into alow-entropy system with stronger hydrogen bonds (H-bonds). TheseH-bonds render a high order around the nonpolar surface, which inturn encourages molecules to self-assemble into larger aggregates [43].In aqueous medium, the H-bonding network extends over the entire

0

10

20

30

40

50

60

70

80

Gases

working temperature 250 °C

200 225 250 275 300 325 35050

55

60

65

70

75

80

85

90

Per

cent

res

pons

e (%

)P

erce

nt r

espo

nse

(%)

Temperature (°C)

CO

250

ppm

CO

50pp

m

C4H

1050

ppm

CH

450

ppm

H2

50pp

m

SO2

50pp

m

Ace

tone

5pp

m

Ace

tone

10pp

m

Ace

tone

50pp

m

Fig. 6. (a) Percent response of SnO2 nanorod sensors for 50 ppm acetone as a

function of operating temperature. (b) Percent response of the single crystalline

SnO2 nanorods (sample S13) for different gas media showing maximum response for

acetone but least sensitivity for CO2.

S. Das et al. / Journal of Crystal Growth 314 (2011) 171–179 177

macroscopic clustering and offers a controllable means to produce andmanipulate structural aggregation by simple long-range ordering ofthe primary nanocrystals [44].

The arrangements of the linear arrays of molecules bend at theedges due to the change in contact angles and the correspondingadhesion forces attach randomly oriented smaller nanocrystallites. So,appropriate changes in the direction and planar coalescence of themolecular groups lead to three-dimensional structures made up ofrandomly oriented nanocrystals. Now with time more and morenanocrystals aggregate on the surface and this leads to thickening ofthe outer shell as shown in Fig. 3(c). The oriented attachment of thesuccessive nanocrystals creates the elongated structures by fusingthe common grain boundaries to achieve the stable arrangements. Thethicker surface of the assembly breaks away from it as nanorods,exposing the immediate inner layers of the assembly, which in turnattach additional nanocrystals. In a repeat cycle of the whole process,later the second and then successive layers also protrude out of theassembly and the entire faceted polycrystalline structures convertedinto single crystalline nanorods. This explains the requirement of thelong synthesis hours (24 h) in order to complete the structuralevolution of pyramidal assembly to single crystalline nanorods inthe present synthesis scheme. It is worthwhile to note that a similarattempt to generate structural variations with aqueous tin (IV) chloridesolution results in irregular and amorphous morphologies (see Fig. 4 ofthe supporting information). We attribute this failure to the absence ofO–Sn–O oxobridges like assembly in aqueous tin (IV) chloride solution,which eventually facilitates as well as initiates the structural formationin the presence of aqueous HMTA solution, as described above.

3.3. Gas-sensing properties

The percent response (S) of the SnO2 nanorod sensors inpresence of different gas was calculated by

S¼ ½ðRA�RGÞ=RA�100 ð6Þ

where RA and RG are the sensor resistance in air and gas (at the sameoperating temperature), respectively. The morphology of thesamples undergone the sensing measurements are shown inFig. 4 of the supporting information. Fig. 6(a) shows the percentresponse of the sensor as a function of temperature. It is revealedthat maximum response for 50 ppm acetone (S¼82%) is obtained at250 1C. So, the subsequent sensing measurements were carried outat this temperature. It is found from Fig. 6(b) that the sensors madefrom single crystalline SnO2 nanorods are highly sensitive towardsacetone whereas their sensitivity is relatively low towards sulfurdioxide, methane, butane, hydrogen, carbon monoxide andcarbon dioxide. This indicates that tin dioxide nanorod sensorscan be used for selective detection of acetone. It is worthwhile tonote that though not illustrated, commercially available micron-sized (around 1 mm) SnO2 (MERCK, Germany) showed poor sensi-tivity towards 50 ppm acetone (S¼42%) and 50 ppm CH4 (S¼27%)under identical sensing conditions. Therefore, we infer that thatmorphology of the thus synthesized nanostructures influences thegas response in the present scheme. In fact, one-dimensionalnanostructures possess large surface-to-volume ratio, makingthem a natural choice for gas-sensing applications [45,46]. More-over, the large surface area of nanorods favours the adsorption ofgases on the sensor and can increase the sensitivity of the devicesbecause the interaction between the analytes and sensing part ishigher. Thus, the size, crystallinity and the concentration of defectsin these one-dimensional nanostructures play major role indetermining the effective gas response of the sensors [47]. Thereare other factors though. For n-type oxides, like SnO2 nanorods, thedeviation from stoichiometry in the form of equilibrium interstitialmetal ions or oxygen vacancies determines the intrinsic carrierconcentration. In nanocrystalline oxides, oxygen vacancy is known

to be the most common defect and is present in the three differentcharge states in the oxides, Vo

0, Vo+ and Vo

2 + , and their reactivitydepends on the operating temperature [48]. Thus, depending onthe variation in the number of oxygen species adsorbed on surfacesand by the change in the number of oxygen vacancies, the reactivityof the SnO2 nanorods varies. In the present study, the maximumresponse in 50 ppm acetone is obtained at around 250 1C, whereas,as mentioned before, the responses for all other listed gases arerelatively low. The selective sensitivities of the present kind can beunderstood in the following manner. We consider that, as a firststep, a ketone (acetone) is adsorbed (also desorbed depending ontemperature) on the sensor surface and the reaction mechanismbetween acetone and adsorbed oxygen species on SnO2, an n-typesemiconductor, may take place as follows [49]:

CH3COCH3(gas)+O�-CH3CO+ +CH3O� +e� (7)

CH3CO+-CH3+CO (8)

CO+O�-CO2+e� (9)

200 225 250 275 300 325 3509.0

9.5

10.0

10.5

11.0

11.5

Temperature (°C)

Res

pons

e T

ime

(Sec

)

0

20

40

60

80

100

120

Rec

over

y T

ime

(Sec

)

Fig. 7. Gas response and the recovery time with SnO2 nanorods based acetone

sensor as a function of temperature.

S. Das et al. / Journal of Crystal Growth 314 (2011) 171–179178

Though the final oxidation products of ketone are CO2, CO andH2O, the reactions may proceed through intermediate steps [50],e.g., the decomposition of acetone may proceed via the formation ofacetyl, formate, etc. The surprisingly low responses for all othergases may be attributed to the lack of surface adsorption of thegases, which fails to produce appreciable changes in the conduc-tance. This lack of adsorption may be due to the absence of catalystson the sensing surface. In general, the presence of catalyst on thesensing surface can result in enhanced dissociation of the mole-cular adsorbates [51,52]. It is known that suitable surface functio-nalization of nanostructures, such as homogeneously Cu dopedSnO2 single nanowire based H2S sensor, can improve sensitivity upto 105 [46,53]. In addition the observed drastic drop in the responseof the sensors above 300 1C can be understood by considering therole of desorption of gas molecules at higher temperatures. It wasreported that for non-stoichiometric or undoped SnO2, the oxygenvacancies are strongly scattering centres, which reduce the mobi-lity of SnO2 nanostructures [54] Therefore, there are also possibi-lities that the gas response at higher temperature is affected by thedefects, grain boundaries and scattering centres. For real lifeapplications of the sensors, the response time (the time requiredfor a sensor to respond to a steep increase in the analyte gas) shouldbe low so that the buzzer of a gas alarm rings during gas leakimmediately. Similarly, the buzzer should not go on ringing for along time (i.e, quick recovery, the time required for a sensor toreturn to baseline after a response to an analyte) after the gas iswithdrawn. In the present case, the response time and the recoverytime (Fig. 7) of the sensors decrease with the increase in operatingtemperature due to faster adsorption–desorption of the gas athigher temperatures.

4. Conclusions

In summary, we have successfully obtained faceted pyramidalstructures of SnO2 using aqueous hexamethylenetetramine(HMTA) solution as structure-directing agents. It was observedthat the structural assembly consisting of randomly orientednanocrystals with sizes of about 2–5 nm. With longer synthesistime of 12–24 h, the faceted structures disintegrated into singlecrystalline SnO2 nanorods with diameter of about 5–10 nm andlength of about 100–200 nm. We found that such morphologicalvariations were achieved in the presence Sn�O�Sn bonds formedby SnCl2 �2H2O in aqueous medium and no such structural evolu-tion was obtained with aqueous SnCl4 �5H2O solution. The nanorodswere employed as gas sensor, which showed high sensitivity

towards acetone and very low sensitivity towards carbon dioxide.For other gases such as sulfur dioxide, methane, butane, hydrogenand carbon monoxide the gas response was intermediate. Thisindicated that tin dioxide nanorod sensors obtained through thepresent synthesis could be used for selective detection of acetone.

Acknowledgements

This work was supported in part by the Priority ResearchCenters Program (2009-0094033) and by the World Class Uni-versity program (R31-20009 and R31-2008-000-10100-0) fundedfrom Korean government (MEST). Authors also thank KBSI, Jeonjubranch for taking quality SEM and TEM images. One of the authors(S.D.) is thankful to Central Glass and Ceramic Research Institute,Kolkata, India, for the gas-sensing measurements.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at doi:10.1016/j.jcrysgro.2010.10.151.

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