Ethanol−Water Mediated Solvothermal Synthesis ... - ACS Publications

© Copyright 2008 by the American Chemical Society

VOLUME 112, NUMBER 16, APRIL 24, 2008

ARTICLES Ethanol-Water Mediated Solvothermal Synthesis of Cube and Pyramid Shaped Nanostructured Tin Oxide Soumen Das,*,† Subhadra Chaudhuri,†,‡ and Sasanka Maji§ Department of Materials Science and Central Scientific SerVice, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032, India ReceiVed: October 24, 2007; In Final Form: February 12, 2008

Randomly oriented orthorhombic tin oxide nanocrystals formed faceted complex structures like nanocubes, nanopyramids, hexagonal discs, etc. SnCl2‚2H2O was used as a precursor in this ethanol-water aided solvothermal synthesis. The growth and self-assembly of primary nanocrystals into complex structures were thought to be aided by thermodynamic energy, viscosity, surface tension, and also by hydrogen bonding, which supported long-range ordering of the nanocrystals. The optical absorbance spectra revealed that the band gap for structured orthorhombic tin oxides is widened from its reported bulk value. The experimentally obtained values of the optical band gap are tallied with the values obtained by using an established theoretical model (Effective Mass Approximation) concerning the interrelation of size and the optical band gap of nanocrystals.

Introduction Solvothermal technique has been proven to be a successful mean to generate nanostructured materials. In fact, this method has been successful in producing numerous structures, like spheroid, cubes, ellipsoids, spindles, rods,1-6 dendrites,7 pyramids, or bipyramids,8 and numerous other shape variations.9,10 Solvothermal method has also been employed immensely for the synthesis and production of nanostructures of crystalline SnO211 and also for obtaining different tin-based compounds.12 The synthesis procedures mainly engulf solvents mediated,13,14 pH depended,4 surfactant assisted,15 and organic template mediated growth.16,17 Research on one-dimensional or multifaceted nanostructural materials, such as nanowires, nanorods, nanocubes, or nanopyramids, is very interesting because of their fundamental * Corresponding author. E-mail: [email protected]. † Department of Materials Science. ‡ Deceased. § Central Scientific Service.

importance and also on account of the wide range of their potential applications in nanodevices.18,19 SnO2 and SnO are known to crystallize under ambient conditions in the rutile and tetragonal litharge structures, respectively. Pure phase SnO2 is an n-type semiconductor with a large band gap of 3.57 eV at room temperature, whereas, experimentally, the gap of SnO has been measured by optical absorption to be around 2.5-3 eV.20 Among these two, SnO2 is widely used as gas sensors, dye-based solar cells, transistors, electrode materials, catalysis, and electrochromic devices.21,22 On the contrary, reports on SnO are few. This is perhaps because rutile SnO2 is very stable, has got high carrier density, and supports enormous concentration of intrinsic and stoichiometryvioleting vacancies, which is correlated with its electrical conductivity,23 felicitating its maximum use as a gas-sensing material. Still, reports on other phases, such as orthorhombic tin oxide, are not rare.24-28 It is seen that the orthorhombic phase occurs when SnO2 is heat treated in a O2-deficient environment under high pressure and that the overall stress of the system is

10.1021/jp800612v CCC: $40.75 © 2008 American Chemical Society Published on Web 03/29/2008

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TABLE 1: Different Synthesis Parameters, Like Amount of NaOH, Temperature, Time, and Volume Metric Ratio of Ethanol and Water for Obtaining Nanostructural Tin Oxide as Well as the Color of the Final Sample and Also the Phase Obtained sample

EtOH:water

NaOH (g)

temp (°C)

time (h)

color

phase

N1 N2 N3 N4 N5 N6 N7

20:20 30:10 10:30 10:30 10:30 40:0 0:40

2.0 2.0 2.0 2.0 4.0 2.0 2.0

200 200 200 200 200 200 200

12 12 12 36 12 12 12

white white white white white gray white

orthorhombic SnO2 orthorhombic SnO2/SnO orthorhombic SnO2/SnO orthorhombic SnO2/SnO orthorhombic SnO2/SnO orthorhombic SnO2/SnO orthorhombic SnO2

reduced on the formation of orthorhombic structure.26 Contrary to this perception Chen et al.27 showed that the high pressure is not an exclusive criterion for obtaining the orthorhombic phase of SnO2, rather he proposed the oxygen exchange reaction mechanism at the grain interfaces for the production of the orthorhombic phase of SnO2. The properties of rutile phase SnO2 have been extensively researched, but we still need to unravel the physical, optical, and electrical properties of orthorhombic tin oxide.29 In our previous attempt30 we obtained the rutile phase of SnO2 having different morphological attributes like spherical and elongated nanoparticles or nanorods by the solvothermal method. We started with SnCl4‚5H2O and attributed the growth of nanocrystals to various surface related thermodynamic energy differences that govern the kinetics of the crystal planes and end up forming the nanostructures. In an attempt to extend the previous work, this time we chose SnCl2‚2H2O as the starting material and used a combination of ethanol (EtOH) and water solvent to examine the evolution of phase and the growth mechanism of tin oxide nanocrystals. We also differentiate the necessary criteria for obtaining pure phase rutile SnO2 or orthorhombic phase of SnO2. Experimental Section Synthesis of SnO2 Nanostructures. The preparation procedure involved is as follows. SnCl2‚2H2O (2.1 g) was taken in a 55 mL Teflon chamber which was filled with required ethanol (EtOH) and water solvent up to 80% of its capacity. The required amount of NaOH was added into it and the solution was stirred for 1 h. The final gray-blackish mixture of the precipitate was transferred to a furnace preheated at 200 °C. The final white or gray product was collected and washed once with deionized water and then repeatedly washed with a mixture of EtOH and deionized water (4:1 volume ratio) for several times. This is because of the observation that the obtained product gets dissolved in plain water. Once thoroughly washed, the final product was dried in air at 100 °C for 2 h. Details of the parameters and synthetic conditions are given in Table 1. Characterization. The phases of the samples were identified by powder X-ray diffraction (XRD, using a Seifert 3000 diffractometer). The morphology and crystalline size of the SnO2 samples were determined by scanning electron microscope (SEM, using Hitachi S2300) and field emission scanning electron microscope (FESEM, using JSM 6700F) accompanied by energy dispersive X-ray analysis (EDAX) attachment for the compositional analysis. The crystal structure was determined by transmission electron microscope (TEM, using JTEM 2010). Optical absorption spectra of the products dispersed in spectroscopic grade EtOH was recorded by a spectrophotometer (using Hitachi U3410) in the wavelength range of 200-900 nm. In the following part of this article we describe the findings in detail. Results and Discussion Phase Study: XRD. The phases of these grown samples are examined through XRD spectroscopy and are shown in Figure

1. For sample N1, most of the peaks are distinguished for the orthorhombic SnO2 with lattice parameters a ) 0.4692 nm, b ) 0.5781 nm, and c ) 0.5238 nm [JCPDS File No. 00-0291484, we denote it as SnO2-A], though the highest peak in this case is indexed to the same phase of SnO2 with lattice parameters a ) 0.4797 nm, b ) 0.5681 nm, and c ) 1.5601 nm [JCPDS File No. 01-78-1063, we denote it as SnO2-B], belonging to the space group of Pbcn. The lattice parameters were determined from the observed peak positions with the help of XRD utility software CELSIZ. In sample N2, other than the orthorhombic phase of SnO2 a smaller intensity of orthorhombic phase of SnO [JCPDS File No. 01-077-2296, we denote it as SnO-A] belonging to the space group of Cmc21 and another crystal phase of SnO [this according to the JCPDS File No. 00-007-0195 has an unknown crystal structure, we denote it as SnO-B] were obtained. Though the presence of the latter crystal phase of SnO (SnO-B) is very minute in sample N2, its presence is appreciable in samples N3, N4, N5, and N6. These last four samples basically are of mixed phase with components from orthorhombic phases of SnO2 (SnO2-A and -B) and SnO (A and B). In most of the samples except N3, N4, and N5 the presence of SnO2-A is dominant. In other cases SnO2-B shows a larger presence. In these samples, it is also observed that between SnO2-A or -B and SnO-A, the presence of the former is prevailing. For sample N7 the peaks are indexed purely to the orthorhombic phase of SnO2-A. Among these, sample N3 prepared in a mixed solvent of EtOH:water with a volume ratio of 10:30 at 200 °C for 12 h shows relatively weak crystalline nature. Other samples on the other hand show a strong preferential growth direction with a smaller full-width halfmaximum of peaks, which would enable us to predict the basic growth mechanism of these samples. Morphology: FESEM and TEM Images. In Figure 2 the SEM images of samples N1, N2, N3, and N4 are shown. Samples N1 and N3 show the formation of pyramid-like structures. N2 samples are pyramidal shaped. N4 samples are mostly cube-like structures, yet additional features, namely patterned hexagonal disc like morphology, can also be observed. N2 samples also show varied size distribution where the smaller structures are identical with sample N1 (see the Supporting Information). Surprisingly, the TEM micrographs of samples N2 (see Figure 3a) and N4 (see Figure 3c) disclosed that these structures are originally made of tiny nanocrystals (see Figure 3b,d) around 2-3 nm in diameter. It is observed that the small nanocrystals are randomly attached to each other and form the larger pyramid (sample N1) and cube (sample N4) structures, as are shown in the Figures 2 and 3. To further ascertain the observed nature of the samples the selected area diffraction pattern (SAED) was taken (inset Figure 3b, d). The concentric rings in the diffraction pattern confirm that the entire aggregation is polycrystalline in nature. For samples N1 (see Figure 3b) and N4 (see Figure 3c) we calculated the lattice planes corresponding to the diffraction rings as (110), (101), and (211) of orthorhombic SnO2 [SnO2-A]. Sample N5 shows the regular cubical structure (see the Supporting Information) as those of

Synthesis of Nanostructured Tin Oxide

Figure 1. The XRD spectra of the nanostructured tin oxide synthesized by solvothermal technique. The samples were prepared in a mixed solvent of ethanol (EtOH) and water by varying their relative volume ratio at 200 °C. The characteristic sharp peaks in the spectra contradict the fact that the nanostructures are made of randomly oriented nanocrystals as observed from TEM micrographs. The reason for this observed nature is explained in the text.

Figure 2. The SEM images of the tin oxide samples are shown: (top) the images for sample N1, N2; (bottom) sample N3 and N4 are shown. For N2 and N4 respectively the structures are pyramid or cube like. For sample N1 and N3 the edges of the structures are not sharp or shapes are not completely defined. For sample N4, other than cube some hexagonal structures are also observed.

sample N4 as the conditions for obtaining them are identical except for the amount of NaOH (see Table 1) being double (4 g). Since we have obtained different nanostructures by simply changing the volume ratio of EtOH and water, hence we may conclude that the basic nature of the solution plays a decisive role in ascertaining the phase and structures of the samples. We have shown in an earlier report4 that the pH standard or the viscosity of the medium determines the growth of nanosturtures or simply encourages the self-assembly of the nanocrystals to grow into larger microstructures. In fact, it is shown30 that the presence of OH- can influence the agglomeration of primary nanocrystals to form larger shapes. According to popular model,31,32 the formation of the mesoscale assembly process may be based on self-organization of crystalline building blocks through the spontaneous coalescence of primary nanoparticles into colloidal aggregates. This way the adjacent nanoparticles are self-assembled by sharing a common

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Figure 3. The TEM images of (a) sample N2 and (c) sample N4. (b) The high-resolution image from the edge of pyramid (N2) along with the SAED pattern indicate that the structure is polycrysatalline made of nanocrystals. A similar case observed for sample N4. In panel d the randomly oriented nanocrystals and the SAED pattern confirm that the nanostructures are polycrystals. The concentric rings are indexed as (110), (101), and (211) of orthorhombic SnO2 [JCPDS File No. 00029-1484).

crystallographic orientation and by docking themselves at a planar interface.33 The driving force for this spontaneous oriented attachment is that the elimination of the pairs of highenergy surfaces will lead to a substantial reduction in the surface free energy.19 Contrary to the assertion of boundary migration, in our study we observed that the primary nanocrystals do not sacrifice their individual boundary surface, but are simply attached to each other to produce sharp edged nanoforms like cube or pyramids. So, our primary task is to explain the reason behind the bending or turning of this self-assembly of nanocrystals at the edges in order to produce the above sharp-faceted nanostructures. This also probably indicates a long-range ordering of the primary nanocrystallites. Claesson et al.34 observed these longer range attractive forces between hydrophobic surfaces. Tsao et al.,35 on the other hand, observed that long-range attraction between a hydrophobic surface and a polar surface is stronger than that between two hydrophobic surfaces. Bawendi et al.36 showed that monodisperse CdSe nanocrystals, capped with hydrophobic ligands having long alkyl chains, could selfassemble into long-ranged ordered crystals with a shape such as pyramid. Figure 4 shows another combination of FESEM images for hexagonal disc-like structures of sample N6. A closer view of the sample revealed that the hexagonal discs are formed by a regular bending of one-dimensional nanorods or by a regular attachment of these nanorods end-to-end fashion. The bottom images in Figure 4 along with the inset figure also act as evidence of the growth pattern of the angular-shaped structures for sample N6. It is also revealed from the optical absorption spectra (discussed later) that these structures are also made of unit nanocrystals which are oriented rather randomly and attached in such a fashion as to retain individual characteristics of the grains. The general morphology of sample N7 is shown

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Figure 4. The FESEM images show the morphology for sample N6. Interestingly, it shows the oriented attachment of one-dimensional nanorod end-to-end fashion. The angular lamella in the top image for sample N6 show the primary orientation of the nanorods to form final larger nanostructures as hexagonal disc or cubes. The bottom image shows some other “T” or “L” shaped lamella.

Figure 5. The N7 sample: (a, b) the FESEM images and (c, d) the TEM images. The images show both nanocrystals agglomerates and a leafy extension. Both parts are made of tin oxide compounds as revealed from the EDAX shown inset of panels b and c, respectively. The inset image of panel c shows the point of origin of the leafy formations.

in Figure 5a. Panels b and c of Figure 5 show some stable but thin leafy extensions. Though the XRD spectra categorically showed that sample N7 is orthorhombic SnO2 (SnO2-A, see Figure 1), to ascertain the composition of these structures

Das et al. conclusively, we undertook the elemental analysis. This is because sometimes carbon-related compounds may form such flake-like morphologies which might avoid detection through XRD spectroscopy. The EDAX (corresponding inset in Figure 5b) shows that these leafy structures are polymorphs of tin oxide. Then again in Figure 5d, the high-resolution TEM image of the bud (that is the starting point of the leafy extension, as shown at the inset of Figure 5c) reveals that this time too, the structures are made of small unit nanocrystals. The EDAX proves that these are compounds of tin oxide as was also proved previously by XRD spectra (see Figure 1). We could not verify the nature of assembly of nanocrystals in the leafy extensions as higher resolution images of these structures could not be obtained due to the damage caused by the highly energetic electron beam. Growth Analysis. To analyze the scheme for the growth of the nanocrystals to complex nanostructures (pyramids, cubes, angular lamellae, etc.) we are open to ideas as enunciated in different extraordinary articles.37-41 Earlier reported observations go like this. As EtOH and water are added together, the water molecules will preferentially bind to the alcohol molecules and the resultant dielectric constant decreases facilatiting stronger electrostatic interaction between the charged segments in the molecules. As reported by Kiely et al.,37 binary colloidal crystals can also be formed based on the van der Waals interactions by precisely controlling the size ratio between two differently sized hydrophobic nanocrystals and by controlling the evaporation rate of solvents. Besides, the hydrophobic interactions play an important role in different surface-interface processes such as self-assembly of molecules on surfaces and the aggregation behavior of micelles. In this aspect the works of Murray et al.38 will be relevant. They showed that suitable surface modification may assist in manipulating the interaction between nanocrystals and thus direct them to organize into defined but complicated structures. It has also been suggested that due to incompatibility, the water molecules surrounding a nonpolar surface rearrange into a low-entropy system with stronger hydrogen bonds (Hbonds), resulting in a high order around the nonpolar surface.39,40 During colloidal crystallizations the entropy may also drive colloidal particles to self-assemble into highly ordered arrays. For pure water the H-bonding network extended over the whole solvent, but in the EtOH and water mixture solvent the H-bond network becomes less dense and forms disconnected macroscopic clustering. These hydrogen bonds encourage molecules to self-assemble into larger aggregates. It has an intermediate binding strength and offers a controllable means to produce and manipulate structural aggregation. Thus hydrogen bonding along with the long-range ordering of the primary nanocrystals perhaps results in a large domain of ordered organizations.41 Figure 6 represents the schematic diagram for the proposed growth mechanism. It starts with simple linear crystallite aggregates. The arrangements of these linear arrays of molecules bends at the edges to form angular lamellae and triangular structures. In fact, Kokkoli and Zukowski in a not so much earlier report42 showed with CH3-terminated surfaces that the change of contact angles and the corresponding adhesion forces can be related by employing solvent mixtures (e.g., water and ethanol). So, appropriate changes in the direction and planar coalescence of the molecular groups lead to three-dimensional construction of nanostructures, as is shown in the bottom image of Figure 4. This adhesion of molecules increases with the increase in water content in the solvent.42 Likewise, riding on favorable synthesis conditions more complex shapes are formed when the triangular planes grow into each other in order to form the facets of pyramids or bipyramids or hexagonal discs. The

Synthesis of Nanostructured Tin Oxide

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Figure 6. The schematic diagram of the growth pattern for the orthorhombic SnO2 nanostructures. The growth starts with the linear arrangements of the primary nanocrystals. Depending on the nature of the edge-ligands the primary nanocrystals bend at the edge to form angular lamellae, or triangles and more complete structures like pyramids. Cubes are formed perhaps in somewhat similar fashion from square lamellae (for a picture of square lamellae see the Supporting Information).

sample is identified as sodium tin hydroxide (Na2Sn(OH)6) [JCPDS File No. 00-24-1143]. This led us to propose that formation of SnO2 initially goes through an intermediate step where the initial reaction occurs supposedly in the following way

SnCl4‚5H2O (s) + H2O (l) f (Sn4+ + OH-) + (H+ + Cl-) (Sn4+ + OH-1) + NaOH f Na2Sn(OH)6

Figure 7. The XRD patterns for solvothermal treated SnCl4‚5H2O and NaOH in ethanol (sample Eth1, Eth2), and ethanol and water mixture solvent (sample EW1, EW2, EW3) at 200 °C for 12 h. Notice the initial formation of sodium tin hydroxide (Na2Sn(OH)6) for the top four cases and also the formation of rutile phase SnO2 in the last case.

formation of cube-like structures starts with square lamellae, with similar unit lamellae joining vertically in order to form cubes. The scheme also includes the formations of tin oxide nanorods as shown in Figure 6. It is claimed that the morphology and the crystallography of the tin oxide depends on the synthesis temperature and pressure.44,45 Experiments27 have also proved that the formation of orthorhombic SnO2 requires low oxygen partial pressure. In our experiment the formation of orthorhombic SnO2 or orthorhombic SnO points either to this deficiency in oxygen ambience or oxygen atomic exchange along with the influence of hydrostatic pressure. This is surprising given the fact that this very reaction condition when repeated with SnCl4‚5H2O following our previous attempt30 ended up producing the rutile phase of SnO2 (see Figure 7). We observed that the rutile phase is not generated in mere basic EtOH solution (NaOH ) 2.0 g) (samples Eth1 and Eth2). On the contrary, for the mixed EtOH and water solvents the pure rutile phase of SnO2 [JCPDS File No. 00005-0467] with space group P42/mnm is obtained if initially the solution is stirred for 1 h before inserting it into the preheated furnace at 200 °C for 12 h (sample EW3). In the first four cases (for samples Eth1, Eth2, EW1, and EW2) the phase of the

Notice the tin remains in the Sn4+ (stannic) oxidation state. If the solution is stirred for a sufficient time then Na+ is stripped from the hydroxide compound to form stable tin hydroxide as

Na2Sn(OH)6 f Sn(OH)2 + (Na+ + OH-1) air, 100 °C, 2 h

Sn(OH)2 98 SnO2 + H2O In other cases where SnCl2‚2H2O is used as precursor, the formation of orthorhombic SnO or subsequently SnO2 is preferred, because high vapor pressure induced formation of the orthorhombic phase perhaps required less atomic arrangements than formation of the rutile phase. The lattice structure of molecules depends on the dynamic nature of the constituent charges. The interaction between the charges may be classified as being due to short-ranged Lennard-Jones potential or longranged Coulomb potential. The optimization process is achieved when along with additional feeding from the crystal field, which has the ability to change the molecular structure, the lattice energy is minimized.46 It is also shown27 that production of orthorhombic phase increases with the reduction of particle sizes, and so we may conclude that the grain size and defect levels in the tin monoxide precursor have a very strong correlation with the fraction of the orthorhombic phase produced during solvothermal synthesis. There are four oxygen species, namely O2, O2-, O, and O-. As the particle sizes are reduced the percentage of oxygen atomic exchange increases because with reduction

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Figure 8. The optical absorbance spectra of nanostructured orthorhombic tin oxide. The spectra showed that the absorbance peak in each of the sample lies in the UV regime of the spectra indicating the presence of nanometric tin oxide in the sample. The minimum wavelength the peak appeared is at 221.9 nm corresponding to a particle with radius 1.3 nm (calculated with effective mass approximation theory47,48).

in particle diameter the surface defects increase which include deficiency of the oxygen atoms at the grain boundary interface.27 The oxygen species (O2- or O-) continuously exchange electrons and thus incorporation of neutral atoms reduces the oxygen vacancies which results in atomic arrangements such that it encourages the formation of orthorhombic tin oxide. Optical Absorption. To find out more about this agglomeration of nanostructures we studied the optical absorption spectra. The powder samples were dispersed in EtOH by ultrasonication for 5 min. The measured spectra are plotted in Figure 8. As the particle sizes lie in the nanometric regime, we expected the absorption edge to lie in the UV region of the spectra. For sample N1, a broad peak is observed around 259.8 nm (4.77 eV). For samples N2, N3, and N6 two absorption edges around 224.1 (5.53 eV) and 265.9 nm (4.66 eV), respectively, are observed. For sample N4 a very prominent absorption edge around 221.9 nm (5.59 eV) and another less intense, broad peak around 267.9 nm (4.63 eV) are observed. For sample N5, the absorption edge is around 254.8 nm (4.86 eV) and for sample N7 a very intense absorption peak is observed at 243.6 nm (5.09 eV). Thus in every case the absorption edge or peak lies in the UV region of the spectra indicating the widening of the optical band gap due to quantum confinement.27,47 Through a simple calculation enunciated by Brus,47 the size-dependent band gap energy for a two-electron system with a slight modification from Babic´ et al.48 can be given by

∆Eg )

p2π2 a2

[

N2

[

1

me*

∞

+

1 mh*

]

+

πe2(a - b)(l + 1) 2a0a[b + l(a + b)

]

1 ∑ ∫0 J02(πx)x2l+2 dx 0

+

2.578  aa

+

e2 (a - b) 4πa 0ab

where me* and mh* are the effective masses of the electron and hole, J0 is the spherical Bessel function of zeroth order, and a and b are the dielectric constants of (rutile) SnO2 (equal to 14.0) and of the EtOH medium (equal to 24.0), respectively. So, a simple numerical calculation showed that particle sizes for the sample corresponding to absorption edges match considerably with those observed from the TEM micrographs. The calculated particle sizes from using the above equation corresponding to the observed band gap are shown in Figure 8 (inset).

In the present study the optical absorbance spectra also revealed that in addition to a well-resolved absorption peak of SnO2 at the UV region of the spectra (as discussed above), another absorption edge is observed especially for samples N3 and N6 at ∼3. 35 eV (∼370 nm). Since TEM micrographs revealed that the assembled particle sizes for all the samples are of nanometric order, we may conclude that the quantum size effect has modified the bulk band gap values of SnO (∼2.75-3 eV) to its present observed value of ∼3.35 eV. This study expicitly relate the quantum confiment effect in orthorhombic SnO2 or SnO and can assists in finding out propeties that relates with the band gap of the samples. The absorbance spectra also proofs that the nanostructures are made of small nanocrystals as otherwise given the size of the nanostructures we would observe another absorbance corresponding to the bulk value (3.57 eV) of SnO2. Now we would like to address the reason behind the intriguing nature of the sharp XRD peaks despite the particles lying in the nanometric region as observed from TEM micrographs. The argument perhaps goes like this. What we see from TEM is a low-angle grain boundary (LAGB), whereas the distinctness of two nanograins can be maintained if the grain boundary is a high-angle grain boundary (HAGB). Thus when we go for XRD, unlike TEM, it sees a larger area of the specimen and most of the grain boundaries in your specimens are LAGB. This kind of situation leads maintaining better coherency among the various grains of the polycrystalline aggregate and the whole specimen behaves like a coarse-grained material from an X-ray point of view. Thus the specimen behaves as a large grain, rendering stronger peaks in the XRD spectra. Conclusion In summary, nanostructures of orthorhombic tin oxides (SnO2 and SnO) were prepared by a solvothermal process by suitably varying the mixture of ethanol and water. Small (∼2.0-3.0 nm) sized nanocrystals are randomly oriented and assembled to form larger pyramid, cube, and hexagonal disc like features. Absorbance spectra of these nanostructures showed the absorption edge lies at the blue region of spectra, which reveals that in the complex structure also the band gap is expanded from its bulk value. This is because the complex structure is made of small nanocrystlas whose optical band gap is modified because of their nanometric size. Effective mass approximation formula relate the optical band gap and the size of the naanocrystals, which confirms that the nanostructures are made of a small nanocrystal assembly that also carries its properties even within the complex structure. Acknowledgment. This work was performed with the support of the Nanoscience and technology initiative programme of Department of Science and Technology (DST), Government of India. One of the authors is grateful to Dr. Puspendu Sahu, at the National Metallurgical Laboratory, India, for some very useful discussion on the topic. The authors are also grateful to Mr. Gopal Krishna Manna, Illustrator, CSS, IACS, India, for very good graphic design. Supporting Information Available: Picture of square lamellae. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zheng, Y.; Cheng, Y.; Wang, Y.; Zhou, L.; Bao, F.; Jia, C. J. Phys. Chem. B 2006, 110, 8284.

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