Materials Research Bulletin 46 (2011) 609–614

Influence of aqueous hexamethylenetetramine on the morphology ofself-assembled SnO2 nanocrystals

Soumen Das a, Dae-Young Kim a, Cheol-Min Choi a, Yoon-Bong Hahn a,b,*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 8 August 2010

Received in revised form 19 November 2010

Accepted 5 December 2010

Available online 29 December 2010

Keywords:

Nanostructures

Semiconductors

SnO2

Chemical synthesis

X-ray photoelectron spectroscopy

A B S T R A C T

The present report details the effects of synthesis time, concentrations of hexamethylenetetramine

(HMTA) and precursor tin (II) chloride solutions on the self-assembly of SnO2 nanocrystals. High-

resolution electron microscopy images revealed that the structures were made of randomly attached

SnO2 nanocrystals with sizes in between �2 and 5 nm. X-ray photoelectron spectroscopy (XPS) showed

that the Sn3d region was characterized by the spin-orbit splitting of the Sn3d5/2 ground state at

�487.6 eV and by the Sn3d3/2 excited state at�496.1 eV, which was attributed to the Sn+4 oxidation state

of the SnO2 samples. We also found that the self-assembly could be achieved only with aqueous tin (II)

chloride solution, and not with aqueous stannic (IV) chloride solution. A plausible growth mechanism is

proposed in order to analyze the distinctive self-assembly of SnO2 nanocrystals in the presence of

aqueous HMTA solution.

� 2011 Elsevier Ltd. All rights reserved.

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1. Introduction

SnO2 is a n-type and wide band gap (3.6 eV) semiconductor [1].This is an industrial material, which is important for its electrical,mechanical and optical properties and can be used in sensors,lithium-ion batteries and catalysts in its different nanostructuralforms [2–9]. The utility of SnO2 in gas sensors or catalysts cruciallydepends on its microstructural features, porosity and on theintrinsic oxygen vacancies. Moreover, the performance of tin oxidedepends on the particle size, compositional characteristics, andshapes, which, in turn, depend strongly on the synthesis process[10–12]. Thus, morphological control of semiconductors is a crucialfactor for fundamental studies of crystal growth and for exploringnew applications of the nanostructures. Researchers, thus, havestudied the suitability of one-dimensional nanostructures, likenanorods, nanowires, nanobelts, nanofibers, as well as highlyfaceted geometries such as pyramids, prisms, and hexagons inmany applications [13,14]. In wet chemical synthesis, thesemorphological variations are achieved by using various surfac-tants, microemulsion or long chained polymers as structuredirecting agents [14,15]. The shape and size of the finalnanostructures also depend on the concentration of precursors

* Corresponding author at: School of Semiconductor and Chemical Engineering,

Chonbuk National University, 664-14 Duckjin-Dong 1 Ga, Jeonju 561-756, South

Korea. Tel.: +82 63 270 2439; fax: +82 63 270 2306.

E-mail address: ybhahn@chonbuk.ac.kr (Y.-B. Hahn).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2010.12.034

and that of the structure directing agents. Studies revealed that thespontaneous coalescence of primary nanocrystallites is driven bythe van-der Waals force, molecular interactions and by the stericeffects [16]. In one example of such phenomena, Yang and Zeng[13] showed that hollow SnO2 octahedra could be obtained by theself-assembly of zero-dimensional nanoparticles. The authorsobserved a novel organizing principle with an underlyingoriented-attachment mechanism in which complex geometricalstructures (e.g., polyhedra) could be built by assembly routes.Based on such findings, different growth models have beenproposed to explain the tendency of molecules to assemble intodefinite geometrical arrangements [17–24]. Therefore, under-standing self-assembly is crucial for it can give us control overspecific device fabrication for desired applications.

The primary objective of the present report is to detail howaqueous hexamethylenetetramine solution (HMTA) can beemployed to obtain faceted nanostructure through self-assemblyof the primary nanocrystallites. In our previous works we usedsolvothermal methods to obtain structural variations of SnO2 [25]and reported the structural evolution from core-shell facetedpyramids to nanorods [26]. In the present work, we describe arelatively simple synthesis process to develop self-assembly inaqueous hexamethylenetetramine solution. We detail the growthof nanostructures under various synthesis parameters and developa plausible growth mechanism to analyze the obtained results. Wefound out that perhaps the hydrogen bonds clustering around thehexamethylenetetramine interacted with the non-polar SnO2

surfaces and formed faceted nanostructures. Interestingly, we

Table 1Details of the synthesis parameters and the morphologies of SnO2 nanocrystals obtained in the hexamethylenetetramine assisted wet chemical synthesis.

Sample SnCl2�2H2O (mM) HMTA (mM) pH Time (min) Relative concentration Shape Size (approx.) (nm)

S1 5 10 3.94 5 1:2 Pyramid 100

S2 5 10 3.94 15 1:2 Pyramid/spherical 200

S3 5 10 3.94 30 1:2 Pyramid 200

S4 5 1 3.06 5 5:1 pyramid 100–200

S5 5 5 3.43 5 1:1 Not regular 200

S6 1 10 5.70 5 1:10 Pyramid 300

S7 10 10 3.20 5 1:1 Pyramid/spherical 50–100

S8 1 1 3.91 5 1:1 Pyramid 400

S9 1 1 3.91 30 1:1 Pyramid 700

S. Das et al. / Materials Research Bulletin 46 (2011) 609–614610

also found out that the self-assembly was possible with aqueoustin (II) chloride solution and not with aqueous tin (IV) chloridesolution. We propose that this is due to the fact that only tin (II)chloride forms Sn–O–Sn oxobridges in aqueous solution, whichassist in the formation of a nanostructural assembly from unitblocks.

2. Experimental

The synthesis process involved is as follows. SnCl2�2H2O wasdissolved in 100 mL of water at room temperature (see Table 1). Ina separate beaker HMTA was dissolved in 100 mL of water at lowtemperature (in ice water bath) (see Table 1). Both solutions werestirred for 5 min. The aqueous HMTA solution looked transparentand the aqueous tin chloride solution was either transparent(1 mM) or, translucent (5 mM, 10 mM). After 20 min, the aqueousHMTA solution was slowly mixed with the aqueous SnCl2 solutionunder constant stirring. Interestingly, in each case, initially themixture solution was transparent, but turned turbid once �60 mLof the aqueous HMTA solution was mixed with the former. Theobtained thick white solution was stirred at room temperature and

Fig. 1. FESEM images of the general morphology of the rutile SnO2 pyramid structure o

�100 nm, (b) sample S2. The edge length is around �200 nm, (c) high resolution image s

(d) assembly of five pyramids (also in sample S1) and (e) stack like assembly in samp

then was transferred to a hot water bath (temperature 90 8C). Aftera few minutes (see Table 1), the powder was collected, washed anddried in vacuum. The as synthesized powder samples were thenused for morphological and other characterizations. Table 1 detailsthe synthesis conditions and sample names.

The crystal structure and the evolution of the morphology weredetermined by field emission scanning electron microscope (JSEM,S-4800, operated at 15 kV), transmission electron microscope(JTEM 2010, operated at 200 kV). X-ray photoelectron spectra(XPS) were recorded with a Thermo K-alpha ESKA System with amonochromatic Al-ka source and a charge neutralizer.

3. Results and discussion

3.1. Morphology: FESEM and TEM studies

FESEM images in Fig. 1(a) and (b) show the general features ofsamples S1 and S2, respectively. The obtained morphologies weremostly pyramidal shaped and consisted of spherical SnO2

structures. High-resolution microscopic image of sample S1 inFig. 1(c) reveals the uneven faces of the obtained pyramids. In

btained in the HMTA assisted synthesis (a) sample S1. The edge length is around

hows the pyramid (sample S1) is made of numerous distinguished nanocrystallites,

le S6.

Fig. 2. (a) and (b) The high-resolution FESEM images of sample S8 show the agglomeration of the individual nanocrystallites on the surfaces of the structures. (c) and (d) High-

resolution TEM images of the pyramid shaped rutile SnO2 nanostructures of samples S1 and S2, respectively. The calculated lattice planes correspond to the (1 0 1), (1 1 0)

with lattice spacing as�0.345 nm and 0.267 nm of rutile SnO2, respectively. The inset figure shows the selected area diffraction pattern form (1 1 0), (1 0 1), (2 0 0) and (2 1 1)

lattice planes. The lattice separation corresponds to (2 0 0) and (2 1 1) planes are calculated as 0.236 and 0.176 nm, respectively.

Fig. 3. The morphological variations with the relative concentration of the HMTA and the tin (II) chloride in the aqueous solution for sample S4–S7. Inset images in (a), (c) and

(d) show the higher magnification microstructural images of the individual nanostructure.

S. Das et al. / Materials Research Bulletin 46 (2011) 609–614 611

S. Das et al. / Materials Research Bulletin 46 (2011) 609–614612

addition, Fig. 1(d) for sample S1shows that as many as five smallerpyramids are attached to each other and Fig. 1(e) indicates stackinglike assembly for sample S6. Fig. 2(a) and (b) shows high-resolutionFESEM images of sample S8. These images clearly demonstrate thatthe faceted SnO2 microstructures are made of randomly attachednanocrystals. High-resolution TEM images of sample S1 and S2 inFig. 2(c) and (d) reveal the lattice structures of these nanocrystals.Calculations indicate (1 1 0) and (1 0 1) parallel lattice planes ofthe rutile SnO2 nanocrystals with lattice spacing as �0.345 and0.267 nm, respectively [27]. Inset, Fig. 2(c) and (d) shows theselected area electron diffraction (SAED) patterns, which expect-edly reveal the polycrystalline nature of the samples. Thediffraction rings correspond to the (1 1 0), (1 0 1), and (2 0 0)and (2 1 1) lattice planes of rutile SnO2. For the (2 0 0) and (2 1 1)planes the lattice separations are calculated as �0.236 and0.176 nm, respectively. Some intense spots on the diffractionrings, especially in Fig. 2(d), indicate marginal diffusion of similarlattice planes.

Fig. 3 shows that the molar concentrations of aqueous HMTAand that of tin (II) chloride solutions also affect the morphology ofrutile SnO2. In fact, the effect of HMTA is very evident in samplesS1, S4 and S5 (see Table 1 for sample descriptions). Fig. 3(a) showsthat the nanostructure (S4) contains a thick central core (see inset,Fig. 3(a)). Fig. 3(b) shows the morphology of sample S5. Fig. 3(c)

Fig. 4. The TEM images show the time evolution (a) 5 min (sample S8) and (b) 30 min (s

aqueous solutions.

reveals that the pyramidal shape of sample S6 with edge length�300 nm and the ring type SAED pattern (not shown, as it isidentical to what are shown in Fig. 2(c) and (d)) reveals thepolycrystalline of the sample. The morphological features ofsample S7, as shown in Fig. 3(d) are identical to those of sample S5(and in both cases the ratio of molar concentrations of HMTA tothat of tin (II) chloride are the same, see Table 1). We notice that(see Fig. 1 of supporting information) with increasing HMTAconcentrations, the obtained structures begin to accumulate morenanocrystallites at the central region, surrounded by less densednanocrystals near the surface. Morphology of sample S8 is shownin Fig. 4(a). The obtained pyramids have an average edge length of�400 nm. Fig. 4(b) shows the time evolution of such structures(sample S9). The reproducibility of all the above-obtainedstructures supports the further study of self-assembly of the rutileSnO2 nanocrystallites in HMTA assisted synthesis.

3.2. XPS Spectroscopy

The oxidation state of the SnO2 nanostructures was determinedfrom the XPS study. Fig. 5(a) shows the typical survey spectrum, aswell as, the characteristics of the O1s and Sn3d regions of thematerial for sample S1. Apart from the weak C1s peak around�286.5 eV, only Sn- and O related core levels are detected in the

ample S9) of the pyramidal structures with 1 mM SnCl2�2H2O with 1 mM HMTA in

525 530 535 540 545

Inte

nsit

y (a

rb. u

nit)

Binding energy (eV)

c

485 490 495 500

Sn+4 3d5/2

Sn+4 3d3/2

Inte

nsity

(ar

b. u

nit)

Binding Energy (eV)

b

0 200 400 600 800 1000

a

OK

LL

Sn3sSn

3p1/

2

Sn3p

3/2

O1S

Sn3d

3/2

Sn3d

5/2

C1S

Sn4s

Sn4pSn

4d

Inte

nsity

(ar

b. u

nit)

Binding energy (eV)

Fig. 5. The oxidation state of the SnO2 nanostructures was determined from the XPS

study. (a) Representative 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 Sn+4

oxidation state of the S1, S3 and S4 SnO2, (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. / Materials Research Bulletin 46 (2011) 609–614 613

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 the Sn+4 oxidationstate of the SnO2 samples (Fig. 5(b)) [28]. The full width of the halfmaxima for these symmetric peaks are calculated as �1.22 and�1.24 eV, respectively. The clear symmetric peaks suggest thatthere are no sub-peaks among them. Therefore, we rule out thepresence of Sn+2 oxidation state for these oxides. The ratio of atomicconcentrations (O/Sn) was quantitatively analyzed by calculatingthe area under the peaks of the O1s and Sn3d5/2 with the help of theLorentzian shape fitting method. The obtained value of �1.89–1.93deviated slightly from the theoretical one and could be due to thepresence of oxygen vacancies present in rutile SnO2 [25]. Fig. 5(c)shows the asymmetric O1s peak characteristics of all the samples.The obtained O1s peaks for the S1 are deconvoluted into twocomponents by fitting it according to the Lorenztian function. Themain peak is identified at�531.5 eV while the shoulder at 532.9 eV.

The main peak is assigned to the lattice oxygen and the shoulder isdue to the oxygen of the Sn–OH bonds [29].

3.3. Growth of self assembled faceted nanostructures

Murray and co-workers [30] showed that with suitable surfacemodifications, the interaction between nanocrystals can bemanipulated in order to direct them to form complicatedstructures. In the present case, the smaller nanocrystallites attachonto each other to form bigger nanostructures. In our under-standing, the key points of influence for the entire growthevolution are: (a) the concentration of aqueous HMTA solution[31–33] and (b) the polymeric chain formed by Sn–O–Sn throughhydrogen bonding of hydroxyl ions (see Fig. 2 of supportinginformation). It was reported by Aladko et al. and Wang et al.[34,35] that at a low temperature HMTA in an aqueous mediumforms a hydrogen bonded netwrok and behaves like polarmolecules [36]. On the other hand, SnCl2�2H2O in an aqueousmedium forms polymeric chains due to hydrogen bondingbetween water molecules [37]. These O–Sn–O bridges are formedfrom structural OH groups belonging to two neighbouring tinatoms resulting in a planar structural arrangment. It appears thatthese oxobridges preferentially form (1 1 0), (1 0 1) and (1 0 0)parallel lattice planes.

We see in Fig. 2 that the outer surfaces of the nanostructures aremainly comprised of nanocrystals having (1 1 0) and (1 0 1)parallel lattice planes of rutile SnO2. It was reported [38,39] thatthe (1 1 0) surface is formed by alternating layers (O(�2)–Sn2O2(+4)–O(�2)) with zero charge and the net dipole of thesurface is zero. In the (1 0 1) surface all tins are five-foldcoordinated, and the equatorial oxygen is absent. The resultingsurface is formulated as (O2(4�)–Sn2(8+)–O2(4�)) and it is alsononpolar [38]. It has been suggested that due to incompatibility,when the nonpolar surface is in close proximity to another polarsurface, the system rearranges into a low-entropy system withstronger hydrogen bonds (H-bonds). These H-bonds render a highorder around the nonpolar surface which in turn encouragesmolecules to self-assemble into larger aggregates [40]. In anaquaeous medium the H-bonding network extends over the wholemacroscopic clustering and offers a controllable means to produceand manipulate structural aggregation by simple long-rangeordering of the primary nanocrystals [31]. In fact, as explainedbefore, the presence of four symmetrically placed N atoms and theoverall cubic symmetry of the HMTA molecule allow the formationof up to four hydrogen bonding interactions: N–H���O or O–H���N,O–H���O, C–H���O and N–H���N [41]. Interpenetration of thesehydrogen bonded patterns creates a three dimensional super-molecular framework [42]. As the scheme goes (see Fig. 3 ofsupporting information), it starts with simple linear crystalliteaggregates. The arrangements of the linear arrays of moleculesbends at the edges due to the change of contact angles and thecorresponding adhesion forces assist in attaching randomlyoriented smaller nanocrystallites. So, appropriate changes in thedirection and planar coalescence of the molecular groups lead tothree-dimensional square shaped lamellae. As hypothesized, thepolymeric chain of Sn–O–Sn form is adsorbed on the surface andgrows. As aqueous HMTA is heated, it hydrolyzes and releases OH�

through the following chemical reactions [26]:

ðCH2Þ6N4þ6H2O ! 6HCHO þ 4NH3

NH3þH2O $ NH4þ þOH�

SnCl2þH2O ! SnðOHÞ2þHCl

h Bulletin 46 (2011) 609–614

Sn2þ þ4H2O ! SnðOHÞ4þ2e� þ4Hþ

SnðOHÞ4 ! SnO2þ2H2O

The released OH� hydroxyl ion in a way affects the nucleationand growth behaviour of the tin oxide nanocrystal. Thus anoptimized OH� concentration leads to a guided growth, ascertain-ing the metal–oxygen–metal bonds in an ordered fashion [25,43].It is seen from Table 1, that the pH value must also play a role instructural evolution, for the best quality pyramid shape wasobtained for a pH value in the vicinity of �4.0. If we increase (ordecrease) the pH of SnO2 from the point of zero charge (4.5–5.0 forSnO2), more surface sites charge up, and will lower the interfacialtension of the system [44,45]. In such a situation, thermodynamiccolloidal stability will result in a considerable lowering of theripening processes. This slow nucleation process is advantageousfor the growth of pyramidal structures [44]. We observed in theFESEM images that all the structures were some form of pyramids,with very scarce presence of spherical SnO2 nanostructures as insamples S2 and S7. Looking at Table 1 the effect of HMTA is obviousas we changed its relative concentrations with respect to precursortin (II) chloride. In each case, we note down the pH values of thesolution. For example, morphological variations in samples S4–S9are quite repetitive; all ended up with some form of pyramids (pHranging from 3.2 to 3.9l with the highest being 5.7, the size of thepyramid also seems larger in the latter case). However, theinteresting observation was regarding the structural completenessthat we observed for samples S8 and S9 with pH largely in therange of 4. In our understanding, and according to learned referee’sobservations [25,43,46], the origin of SnO2 structures is acombined effect of the concentration of OH� in the solution, theconcentration of HMTA and the presence of aqueous tin (II)chloride solution. At the same time, we must admit that anelaborate study in the line of the work by Testino et al. [47] isneeded to understand the growth mechanism of SnO2 nanos-tructures and to unravel the roles of different factors includingtemperatures, ionic concentrations, pH and thermodynamicparameters.

4. Conclusion

In summary, in this work we demonstrated that aqueous HMTAsolution could influence the structural growth of SnO2 nanocrys-tals. We detailed how in a rapid synthesis process nanocrystalswere randomly arranged to form larger faceted aggregatesdepending on the concentration of HMTA and synthesis time.Interestingly, such self-assembly was not possible with aqueoustin (IV) chloride solution. As the importance of SnO2 in the presentresearch trend is still crucial for a number of factors, our researchcan assist in better understanding the mesoporous, self-assembledgrowth of nanocrystals for desired applications. However, anelaborate study is necessary to understand the influences ofdifferent factors including temperatures, ionic concentrations, pHand thermodynamic parameters on the structural evolution.

Acknowledgements

This work was supported in part by Priority Research CentreProgramme through the National Research Foundation of Korea(2010-0029707) and by the World Class University Programme(R31-2009) funded by the Korea Ministry of Education, Science andTechnology (MEST). Author also thank Jeonju branch of KBSI forSEM and XPS analysis. Authors also extend special thanks to Mr.Jong–Gyun Kang of TEM Lab, Centre for University-wide ResearchFacilities for recording excellent microscopic images.

S. Das et al. / Materials Researc614

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in

the online version, at doi:10.1016/j.materresbull.2010.12.034.

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