Growth Kinetics of Self-Assembled Indium ... - ACS Publications

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Growth Kinetics of Self-Assembled Indium Hydroxide and Oxide in Electrolytic Alkali Halide Solution Vishal D. Ashok and S. K. De* Department of Materials Science, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032, India

bS Supporting Information ABSTRACT: Introduction of a critical amount of different alkali halides in an otherwise identical aqueous synthesis condition was found to produce different morphologies of indium hydroxide from an In metal source. In this study, synthesis in KCl medium leads to polycrystalline bundled rodlike morphology whereas KI medium leads to a singlecrystalline sheetlike morphology of indium hydroxide, which upon being annealed at 250 C transforms to oriented mesocrystals of cubic and hexagonal indium oxide phases, respectively. The difference in the morphology is likely due to the change in the pressure at the interface of the solution and the product, which is controlled by the structure and interactions of the monomers, similar to that observed for block copolymers. The evolution of the polymorphs comes with some interesting thermodynamics of decomposition, which is evident in the thermal studies. This study also indicates the role of crystallinity and dimensionality of indium hydroxide in the formation of different polymorphs of indium oxide.

’ INTRODUCTION Manipulation and control of semiconductor nanoforms, particularly oxides, has been a subject of scientific investigation for the past few decades with a recent inclination toward selfassembled growth as it leads to higher structural and chemical stability.1 The oriented nanoforms show unique physical and chemical properties which are enhanced with the introduction of porosity and could lead to novel properties due to crystallographically oriented interfaces.2,1c Indium oxide (In2O3), which exhibits hexagonal (h-In2O3, space group R3c, no. 167, a = 5.491 Å, c = 14.526 Å, Z = 6) and cubic (c-In2O3, space group Ia3 no. 204, a = 10.126 Å, Z = 16) polymorphs, is a widely used semiconductor due to its electric, optical, photovoltaic, catalytic, magnetic, gas, and biological sensing properties especially after the recent modification in bandgap to 3.1 and 3.34 eV for c-In2O3 and h-In2O3, respectively.3 A wealth of information on the growth of indium oxide and hydroxide nanorods, nanotubes, cubes, needlelike microspheres, nanopyrimids, and oriented supercrystal structures using various chemical routes can be obtained in the literature.4,1a From recent studies, several novel mechanisms have been revealed. Hydrolysis and condensation or alcoholysis have been recently highlighted in solution systems by Peng et al. to generate oriented structures through “limited ligand protection”.4o The morphotropic reconstructive transformation mechanism in dehydroxylation reactions which is followed in the In(OH)3 f In2O3 transformation is not completely realized. c-In2O3 can be r 2011 American Chemical Society

transformed to h-In2O3 by heating at high temperature (>1000 C) under a pressure of 65 kbar. h-In2O3 has been prepared directly by the dehydration of metastable InOOH, and by methanol-mediated solgel processes.5 In addition, h-In2O3 can also be stabilized by the substitution of some indium ions with smaller metallic ions. Very recently it has been shown that the formation of cubic and hexagonal phases depend on the heating rate and temperature of annealing.1a,5c Farvid et al.5c by monitoring the reaction tempaerture observed that the InOOH phase is a common intermediate for h-In2O3 and c-In2O3. They showed by contolled synthesis that the phase formation primarily depends on the crystallite size of In2O3, with h-In2O3 attaining stability below 5 nm. It was also demonstrated that the formation of c-In2O3 follows the phase transformation sequence, In(OH)3 f InOOH f h-In2O3 f c-In2O3 with the increase of reaction temperature and time. The metastable h-In2O3 is an intermediate during the growth of c-In2O3 nanocrystals. However, the exact mechanism for the formation of the two polymorphs of In2O3 is still not clearly understood. The use of ionic liquids for the synthesis of nanocrystals helps to increase nucleation, leading to generation of large number of small sized particles due to low interfacial energy, as the presence of free ions results in a lowering of the interface tension.6 The Received: December 15, 2010 Revised: March 31, 2011 Published: April 22, 2011 9382

dx.doi.org/10.1021/jp111917m | J. Phys. Chem. C 2011, 115, 9382–9392

The Journal of Physical Chemistry C

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ionic liquids also show inherent structures in the liquid state that could be exploited in synthesis and other various applications. NaCl and other halides are seen in many chemical syntheses as a byproduct but the role of these halides in the growth mechanism is often neglected. In this manuscript, we describe the preparation of templatefree self-assembled mesoporous indium oxide nano/microstructures through a soft chemical process and study the effect of some alkali halides (AX, A = Li, Na, K; X = Cl, Br, I) on the growth of structures and phases in these electrolytic solutions. A correlation between synthesis conditions, morphology, and crystallographic structure is indicated.

’ EXPERIMENTAL SECTION The synthesis method followed was a soft chemical process with urea used as an oxidizing agent to indium nitrate formed by dissolving indium in concentrated nitric acid (HNO3). All the chemicals were obtained from Merck, India, and were of GR grade; the chemicals were used as received, and no further purification was performed. The oxidation process of urea is well-known.7 In a typical reaction, indium was dissolved in 69% HNO3 such that the molar ratio of In:HNO3 is 1:6. Nitric acid is used in excess for easy and complete dissolution of indium metal. This solution was diluted using a premixed aqueous urea solution to result in a final molar ratio of of 1:6:6:130 for In:HNO3:NH2CONH2:H2O. A concentration of urea was selected so that it could neutralize the excess nitric acid. This solution was heated at 95 C under stirring for 80 min. After 1 h, a white precipitate appeared in the solution, which was allowed to age for 20 min. This precipitate was separated and washed with water and ethanol under centrifugation. These samples were later vacuum-dried and annealed at 250 C for 8 h. The synthesis was modified by dissolving various alkali halides (AX, A = Li, Na, K; X = Cl, Br, I) into the urea solution prior to the dilution, with a view to study the effect of alkali halides on the growth mechanism. For simplicity, the as-prepared samples were labeled InU, InKCl, and InKI corresponding to samples prepared using aqueous urea solution, ureaKCl solution, and ureaKI solution, respectively, and the annealed samples were labeled as InUA, InKClA, and InKIA. The as-prepared and annealed samples were characterized using X-ray powder diffraction (XRPD Bruker D8Advanced with Cu KR) for phase identification, field effect scanning electron microscopy (FESEM JEOL JSM-6700F), and transmission electron microscopy (TEM JEOL JEM-2010) for morphological and phase analysis. The minimum beam width used to generate the SAED (selected area electron diffraction) was 50 nm. The thermal behavior was analyzed by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) using an SDTQ600 instrument (TA Instruments) and by temperature-modulated differential scanning calorimetry (MDSC) using a Q2000 instrument (TA Instruments). ’ RESULTS AND DISCUSSION Basically three major morphologies are observed in the products prepared under different conditions. Agglomeration of spherical particles, oriented bundles of nanorods, and stacked nanosheet-like structures appear for samples prepared with urea solution (InU), 1 M aqueous KCl (InKCl), and 1 M aqueous KI solution (InKI) respectively, as shown in FESEM images (Figure 1). The particles are on the order of 10 nm. Sheetlike

Figure 1. FESEM images of the product obtained by reaction of indium in HNO3 with (a) aqueous urea solution (InU), (b) 1 M KI in aqueous urea solution (InKI), and (c) 1 M KCl in aqueous urea solution (InKCl).

structures show a definite morphology with a width of about 250 nm and a length of 1 μm. Each bundled rodlike structure is 100 nm in width and 11.5 μm long. TEM micrographs of InU and InUA (Figure 2a and Figure 2b, respectively) provide additional detail. A diffused ringlike feature is observed in the case of SAED patterns corresponding to unannealed InU samples. A small region of crystallinity is also observed in the HRTEM image. The crystalline region might be due to the bombardment of an electron beam on the sample which might decompose the sample. The inset of Figure 2b shows the SAED pattern of these annealed particles (InUA), 9383

dx.doi.org/10.1021/jp111917m |J. Phys. Chem. C 2011, 115, 9382–9392


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Figure 2. TEM image of the (a) unannealed urea-based precursor (InU), and (b) In2O3 prepared by annealing the urea-based indium precursor (InUA) formed by the oxidation of indium nitrate in urea solution. Insets: SAED pattern showing h-In2O3 diffraction pattern; HRTEM showing the individual spherical particles of the sample.

which shows a diffraction pattern corresponding to h-In2O3. These particles are arranged such that the agglomerations appear to have a polycrystalline arrangement as evident in the SAED. The HRTEM (high resolution transmission electron microscopy) image shows an individual well-crystalline grain with size on the order of 12 nm. A minor tendency for short-range oriented agglomeration among these grains is also noted. In the presence of 1 M aqueous KCl, the as-prepared products (InKCl) show bundled nanorod-like morphology with an average width of 6080 nm, consisting of smaller rods of diameter ca. 10 nm as shown in the TEM images (Figure 3). A closer look at the HRTEM image reveals the polycrystalline nature of the rods, suggesting that the growth is not epitaxial and is controlled by the diffusion of its ingredients. The well-defined shape appears as if

the growth occurs due to an external medium such as a soft template. When annealed at 250 C (InKClA), they assemble into spherical particles of c-In2O3 oriented along the [440] direction to form a nanorod bundle as observed in the SAED and HRTEM micrographs (Figure 3b). This is the nonpolar direction with the least surface energy as observed by Hao et al.8 When annealed above 300 C, their structural stability depends on the heating rate. They remained intact at a heating rate of 10 C/min but broke into smaller rods upon heating at a higher rate. This may result from the expulsion of water molecules from the rodlike structures, destabilizing the structure. In the presence of 1 M aqueous solution of KI (InKI), the majority of the as-prepared products show sheetlike structures with a clear single-crystalline HRTEM and FFT pattern corresponding 9384



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Figure 3. (a) TEM image of an indium hydroxide nanorod bunch prepared by carrying out the oxidation reaction in KCl solution. Insets: SAED shows the amorphous nature of these samples; HRTEM shows the polycrystalline surfaces. (b) TEM image of indium oxide nanorods obtained by annealing the In(OH)3 nanorod bunches. Insets: SAED pattern corresponding to cubic indium oxide oriented along the [440] direction, consisting of spherical particles as shown in HRTEM.

to In(OH)3 as depicted in Figure 4a. These sheets broke up into particle- or weblike porous (dendritic) structures when annealed at a temperature of 250 C (Figure 4b). The SAED patterns reveal a structure corresponding to h-In2O3 (JCPDS file no. 22-0336, space group R3c [167]) indicating a transformation of In(OH)3 to h-In2O3, which is against the generally believed metastable structure InOOH transformation into h-In2O3, as the presence of In(OH)3 hinders the stabilization of h-In2O3.9 The present observation also suggests a possibility of a direct conversion mechanism similar to the transformation path for cubic phase formation (In(OH)3 f c-In2O3 without any intermediate phase) as observed by Schlicker et al.10 However, the possibility of an amorphous transient species

of InOOH is not ruled out at this temperature (250 C), as the transformation is very fast. Figure 5 shows the thermogravimetric (TGA) variation of InU (Figure 5a), InKCl (Figure 5b), and InKI (Figure 5c). A monotonous decrease in the mass is observed in the precursor as the temperature is increased, with a maximum loss appearing between 275 C and 300 C in samples with similar features at these temperatures. This corresponds to the temperature of In2O3 formation through decomposition of the precursor. The mass loss of InU is more than that ofInKCl or InKI. The features of DTA (differential thermal analysis) in the three samples remain the same except for a peak appearing above 275 C where the indium oxide formation takes place. There is an endothermic 9385



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Figure 4. TEM image of (a) indium hydroxide sheetlike morphology formed by carrying out the oxidation reaction in KI solution. Insets show singlecrystalline nature of the In(OH)3 phase as seen in FFT and HRTEM images. (b) Weblike morphology obtained by annealing the above sample. Insets show a single-crystalline nature h-In2O3 phase as observed in the SAED and HRTEM.

peak at a lower temperature around 220 C corresponding to the cooling due to loss of mass, probably arising because of decomposition of NH4NO3 that might form in the reaction.1a The mass loss of InKCl and InKI (81%) is found to be very close to that of In(OH)3 to In2O3 (83%), suggesting that the two as-prepared samples were In(OH)3. This also indicates that the addition of KCl or KI to the solution merely changes the chemical environment of the growth process, and the salts do not seem to form a complex with the final structure. It may also be inferred from the TGA that the amount of indium oxide per gram of precursor increases with the addition of an electrolyte to the solution, suggesting densification of indium in the precursor. This is expected because of the suppression of the electrostatic effect arising from the free ions in the electrolyte.

From the XRPD pattern it is observed that the InUA (Figure 5d) and InKIA (Figure 5e) show h-In2O3 (with a = 5.462 Å, c = 14.446 Å, and a = 5.466 Å, c = 14.476 Å, respectively) formation whereas InKClA (Figure 5f) forms c-In2O3 (with a = 10.103 Å as fitted using the MAUD (material analysis using diffraction) program). These observations can be correlated to the previous DTA measurement, as the cubic indium oxide is formed along with a peak at around 300 C in the DTA plot. This shows that the exothermic peak seems to correspond to the discontinuity in the specific heat of the sample which arises during a second-order phase transition of amorphous precursor to c-In2O3 as also seen in the SAED patterns in Figures 3 and 4. The exothermic peak area in the DTA plot is proportional to the ΔH of the system, which depends on the conditions of synthesis.11 The average 9386



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Figure 5. DGA/DTA plot of (a) urea-based (InU), (b) KCl þ urea-based (InKCl), (c) KI þ urea-based (InKI), and XRPD patterns of (d) annealed InU (InUA) showing the h-In2O3 phase, (e) annealed InKCl (InKClA) showing the c-In2O3 phase, (f) annealed InKI (InKIA) showing the h-In2O3 phase with c-In2O3 and h In2O3 given below as a guide. This c-In2O3 phase is accompanied by an exothermic peak at around 300 C in the DTA plot.

crystallite size obtained by applying Scherrer’s equation for c-In2O3 and h- In2O3 using peaks corresponding to [222] and [110] directions, respectively, is 20.7 and 21.03 nm, which is in agreement with the TEM analysis. This also rules out the role of crystallite size in the final phase formation. In order to further investigate the phase formation, temporal development of the phase was studied using XRPD for samples annealed for different times. Figure 6 shows the XRPD pattern of InKCl and InKI annealed for 5, 10, and 15 min. The initial peaks observed in the precursor are that of ammonium nitrate formed in the reaction of nitrate with urea. When InKCl is annealed, it turns into cubic In2O3, and when InKI is annealed, it transforms into h-In2O3, both within 15 min of annealing. The slight indication of hexagonal In2O3 is also seen as a broad base between 30 and 32 after 5 min of annealing as stated by Farvid et al.5c This also suggests that the phase change can be described as a morphotropic transformation undergoing a reconstructive phase transition.10 The annealing conditions were further simulated in MDSC with a vision to study the nature of the transformation leading to different polymorphs. An isothermal and thermal profile of heat flow was taken for InKCl and InKI samples. The behavior of the heat flow in any thermodynamic system undergoing a transformation subjected to varying temperature could be expressed as a linear combination of components that depend on the rate of change of temperature and on the

absolute temperature, viz.;

dQ dT ¼ Cp þ Fðt, TÞ dt dt

ð1Þ

where Q is the heat transfer, t is the time, T is the temperature, Cp is the reversing heat capacity arising from the molecular motion (vibrational, rotational, translational; hence, relating to the degrees of freedom), and F(t,T) is the heat flow arising as a consequence of kinetically hindered events. In MDSC, the samples are subjected to varying temperature given by T ¼ T0 þ bt þ B sinðωtÞ

ð2Þ

where T0 is the initial temperature, b is the underlying rate of heating, B is the amplitude of modulated heating, and ω is the frequency of the applied modulation. This allows us to convolute the rate-independent and -dependent terms of a thermodynamic process along with the variation of the heat capacity in an isothermal system.12 Figure 7a and 7b shows the isothermal part of the heat flow vs time at 250 C for 45 min carried out using a modulation amplitude of 1 C and time period of 60 seconds. A clear difference in the behavior of the heat flow and the heat capacity are observed during the formation of the cubic and hexagonal In2O3 phase. An exothermic peak appears in the heat flow curve for c-In2O3 9387



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Figure 6. Temporal development of the phase to (a) h-In2O3 for precursors prepared in 1 M KI þ urea solution (InKI), or to (b) c-In2O3 for precursors prepared in 1 M KCl þ urea solution (InKCl).

formation (Figure 7a) along with a bulging curve in the reversing heat capacity (Figure 7c) of the sample from 10 to 20 min whereas a monotonous linear increase in the heat flow is observed during h-In2O3 formation (Figure 7b) along with an exponential change in the reversing heat capacity (Figure 7d). As Cp is the energy required to increase the temperature of the system by 1 C, an exponential variation in the heat capacity is expected as observed for InKI (Figure 7d). The bulging nature in the Cp for the precursor prepared in the presence of chloride ions implies that there is a larger amount of energy required to raise the temperature of the system by 1 C. This along with an exothermic peak indicates that the heat evolved results from the trapping of the energy due to an increase in the degrees of freedom of its ingredients. This might be due to the polycrystalline nature of the initial precursor. A phase lead in the heat flow with respect to temperature is also observed during this transformation. This is also supported by the fact that the change in the mass in both InKI and InKCl is the same, and there is no presence of any intermediate phase as seen in the temperoral evolution of XRPD. If reversing heat capacity is considered proportional to the amount of initial and final product present in the sample, then the time (t)-dependent fraction of initial precursor converting to indium oxide could be given by f ðtÞ ¼

Cp ðtÞ Cp ð¥Þ Cp ð0Þ Cp ð¥Þ

ð3Þ

where Cp(t) is the observed time-dependent heat capacity, Cp(0) is the initial heat capacity of the precursor, and Cp(¥) is the final heat capacity of the product. This fraction shows a characteristic sigmoidal nature that follows the KolmogorovJohnson MehlAvrami (KJMA) equation as given by f ðtÞ ¼ 1 expð kt n Þ

ð4Þ

where f is the fraction converted, k is the constant of proportionality, and n is the exponent which depends on the combination of nucleation rate, rate of grain formation (seeding), nature, and dimensionality of growth, i.e., with a 0 seeding rate, n takes a value of 1, 2, and 3 for one-, two-, and threedimensional interfacial growth, respectively; under a constant nucleation rate, the value of n changes to 2, 3, and 4, and under a diffusion-controlled growth, the value of n changes to 1/2, 1, 3/2, etc. The value of n can be obtained from the slope of the ln(ln(1/(1 f))) vs ln(t) plot.13 Figure 8 shows the plot of the Avrami coefficient vs ln(t) obtained by differentiating the plot of ln(ln(1/(1 f))) vs ln(t) using Origin software. The local scattering in the data points causes the fluctuation in the values of the Avrami coefficient observed in the graph. The graph for InKCl could be broadly divided into three regions. The first region ranges up to 6.2 where the value of the Avrami coefficient is large and constantly decreasing. This might be due to a large initial seeding rate. The second region ranges from about 6.2 to 6.7. Here the value of 9388



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Figure 7. Isothermal modulated differential scanning calorimetry (MDSC). (a) Heat flow vs time for precursors prepared in 1 M KCl þ urea solution (InKCl). (b) Heat flow vs time for precursors prepared in 1 M KI þ urea solution (InKI). (c) Reversing heat capacity vs time for InKCl. (d) Reversing heat capacity vs time for InKI.

the coefficient hovers around 2, suggesting a site saturation scenario where the grain growth is taking place with a zero or constant seeding rate. The third region ranges from 6.7 to 7 where the value of the coefficient rises dramatically before again decreasing to around 0. This behavior may be assigned to the impingement effect where the nucleation process acclerates inward because of strain developed in the outer crust since the growth cannot proceed anymore. The behavior of the InKI sample leading to h-In2O3 is largely a featureless with a value of the Avrami coefficient of about 0.5, suggesting a one-dimensional diffusion process on pre-existing nucleation sites (seeds) that leads to the growth of webbed features seen earlier in the FESEM and TEM images. The nucleation might be taking place on the edges of the sheetlike morphology. The values of the Avrami coefficients were also observed to increase with the increase in the frequency of modulation. In order to ascertain the role of cations in the morphological development, KCl was replaced by LiCl and NaCl in the synthesis process, keeping the same molarity. No significant change in the bundled rodlike morphology was observed at 1 M concentration. This morphology was observed in the samples prepared in a KCl solution with a concentration as low as 0.005 M. At a lower concentration (0.001 M) of KCl, flakelike structures were observed as shown in the FESEM micrograph of Figure SI1 (Supporting Information). However, these flakes were of irregular shape and sizes.

In order to understand the role of urea, the KCl-based reaction mixture without urea was placed in ammonia gas atmosphere after being stirred 1 h at room temperature and at elevated temperature. The rate of reaction was controlled by the absorption of ammonia into the aqueous solution. At room temperature, a uniform spherical particle precipitate was obtained, whereas at elevated temperature, slightly elongated structures appeared. However, the length of these elongated structures were less as compared to the ones prepared in urea solution. This indicates that the hydrolysis of indium nitrate is important in the formation of these elongated structures. Quantum chemical calculations14a and pump probe measurements14b demonstrated that a water molecule makes one hydrogen bond with the carbonyl oxygen and another hydrogen bond with the cis hydrogen of the urea molecule. The doubly hydrogen bonded water molecule in the solvation shell of urea is strongly immobolized. The hydrolyzed indium would spend more time in this vicinity hence increasing the probability of forming an ordered structure, effectively increasing the coherence length of the structures formed under this condition. Hence, the role of urea is probably to help increase the coherence length of the structure by adding stability to the dynamics of water molecules surrounding the urea molecules14 (see Supporting Information Figure SI2). However, in order to study the actual mechanism, some detailed measurements need to be carried out that would involve time-resolved 9389



Figure 8. Variation of the Avrami coefficient with time (ln t). (9) Precursors prepared in 1 M KCl þ urea solution (InKCl). (O) Precursors prepared in 1 M KI þ urea solution (InKI).

investigations of the structure in the solution, which is beyond the scope of the present study. The morphologies observed here show a striking similarity to the structures formed by the self-organization of sodium dodecyl sulfate (SDS) solution in water in the presence of NaCl and the conformationally asymmetric block copolymers, where the structures depended on the concentration of the minority polymers and their mutual interactions.15 The structure of the minority polymer shows a disordered nature at all concentrations when the repulsion between the two polymers is low. At higher repulsion forces, the minority polymer tries to assume a shape so that the filling fraction is in accordance with the concentration. The shape changes from spherical to cylindrical to laminar structure as concentration is increased. This transition also depends on the lengths of the polymers involved. Based on the above observations, a growth mechanism of the precursor and its conversion into structured indium oxide can be summarized as follows. The morphological evolution can be easily explained by the block copolymer phase diagram depicted by Forster et al. and Matsen et al.15dfIn the absence of any electrolyte the interaction among the indium nitrateurea complex is weak, which leads to the formation of spherical particles in a disordered arrangement. In the presence of an electrolyte a decrease in the solvation of In(NO)3 takes place by a salting out process along with a segregation in the initial grains formed by shielding the electrostatic forces, both in effect changing the FloryHuggins interation (χN) parameter. Thus, upon addition of an electrolyte (KI), the segregation of these complexes takes place effectively, increasing the interations that lead to the formation of more space-filling laminar structures. Similar structures are observed in the presence of Cl ions but for a concentration lower than 0.005 M, as the charge density of Cl ions is much higher than that of I ions owing to its smaller ionic radius. Upon an increase in the concentration of Cl ions, the interaction of the indiumurea complex with the solution increases, which overcomes the force of segregation, leading to a prismatic or bundled rodlike nature. The presence of Br ions produces morphologies similar to that of Cl ions (see Supporting Information Figure SI3). This may be attributed to the fact that

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Figure 9. Schematic diagram representing the change of shape due to the change in the interfacial strain (a) in the presence of chloride ions, when the solution pressure is greater than the internal pressure of the rod, and (b) in the presence of iodide ions when the solution pressure is equivalent to the internal pressure of the product. These pressures are controlled by the virial coefficients, which arise from the monomer structure and the interactions of the solvent molecules.

bromide and chloride ions with hydrolyzed indium in aqueous medium are isostructural.16 This also points to the involvement of molecular structure in the morphological development. The poor crystallinity or polycrystalline nature of In(OH)3 in the case of preannealed bundled rodlike structure seems to result from the short-range interaction at the product solution interface. This result is supported by comparing the rough surface of elongated structure grown in KCl solution when stirred in NH3 atmosphere after heating for 1 h with the smooth nature of the surface of the sides of the nanorod prepared using urea. The polycrystalline nature may also arise from the structural strain induced over the surface. An alternate explanation for the morphological evolution of In(OH)3 is that hydrolyzed indium in the aqueous medium behaves as tiny micelles which, in the presence of KCl, undergoes self-organization from suppression of the electrostatic effect, causing the second virial coefficient to take over which is a function of interactions and pressures and depends on the structure of the monomer. This also results in the decrease of the effective molecular weight because of the interactions giving rise to the elongated structure in an attempt to equalize the interfacial pressure to minimize surface to volume ratio17 (see schematic diagram in Figure 9). Note that the interfacial pressure in the block copolymer depends on the molar concentration of the polymers, leading to a structural evolution of the minority polymer. Upon being annealed, the rodlike structures (InKClA) show the appearance of spherical particles, all oriented along a crystallographic direction (440), whereas the bundled sheets tend to form dentritic monolithic network structures. This may be explained using the model proposed by Son et al.18 where the shape of the structure changed when the size of the reaction zone was greater than crystal size. When the reaction zone is less than the crystallite size, the structure remains the same, except for the transformation taking place layer by layer as shown in Figure 10a. Hence, the formation of the spherical particles is an indication that the size of the reaction zone site is greater or equal to the size of the initial crystal.18 This is preceded by a growth mechanism, as stated by Alivisatos et al., where the nucleation takes place in the defect region produced by the boundaries of polycrystalline indium hydroxide.19 The size of these nanospheres depends on 9390



Figure 10. Schematic representation of the (a) growth of cubic indium oxide mesocrystals from the polycrystalline rodlike structure. This transition is due to the reaction zones (yellow region) being smaller than the nanorod bunches and greater than the constituent crystalline zone, leading to cylindrical rods consisting of spherical particles (in green). These particles in the process of formation orient themselves in the direction of minimum surface energy. (b) Growth of weblike hexagonal indium oxide structures form the single-crystalline indium hydroxide nanosheets where the reaction zone is smaller than both the crystalline size and the sheet size. This leads to a weblike dentritic structure within the reaction zone, which is the characteristic feature of a rapidly cooled molten liquid where the composition volume is changed.

the separation of the nucleation sites and the Gibb’s free energy of the formation and self-diffusivity of indium oxide.11 When the sheetlike structures are annealed, a continuous nature of structure is observed. This suggests a growth mechanism initiated from the surface, which is the case when the crystal size is larger than the size of the reaction zone.18 The overall structure of the material is conserved, leading to a continuous interlinked network as seen in the FESEM and TEM micrographs. A brief discription of the growth of oriented nanocrystals and weblike structure is given in the schematic diagram in Figure 10b. The effect of the crystallite size of the precursor on the phase of the end product can be ruled out, as both InU and InKCl have similar crystallite sizes that give different phases. The role of strain induced by the halides on the phases can be ignored because the samples were purified by washing before annealing. No indications of the presence of halides are seen in the TEM micrographs or TGA studies. The phase transformation of initial precursor to the final h-In2O3 or c-In2O3 seems more likely due to the shape of the initial precursor. The shape of the precursor could also incorporate strain into the sample, which might lead to the formation of the polymorphs. Our comprehensive studies suggest the dependence of phase on the growth kinetic or dimensionality as indicated from the Avrami coefficients. This can be associated with the kinetic energy of the individual entity (i.e., whether the growth of the transformation has two or three degrees of freedom). Particles that diffuse over the surface (with lower energy or Avrami coefficient) tend to arrange in h-In2O3 whereas particles that diffuse into the bulk (with higher energy or Avrami coefficient) tend to arrange into c-In2O3.

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The phase formation of c-In2O3 and h-In2O3 can also be correlated to the crystalline nature of the precursor. Rodlike In(OH)3, being polycrystalline, consists of several boundaries or interfaces which lead to the trapping of the evolved heat in the material, giving a second-order phase transformation-like appearance in the Cp vs time profile. Alternatively, the samples prepared by annealing sheetlike In(OH)3 give rise to a free transformation, as evident from the exponential nature of the Cp vs time with an Avrami coefficient of 0.5. However, whether the formation of cubic and hexagonal In2O3 is an intrinsic property of the rodlike and sheetlike morphology resulting from the initial arrangement of the molecules in the solution (i.e., structural strains induced on the surface) or dimensionality (i.e., two degrees of freedom and three degrees of freedom of the transforming entity) or the crystallinity (i.e., monocrystalline and polycrystalline nature of the unannealed sample) of the unannealed precursor is not confirmed. What is observed is a clear difference in the behavior of the kinetic energies of the entities that form cubic and hexagonal In2O3. In order to show the usefulness of the produced nanorod bunches, porosity and pore size measurements are carried out using nitrogen adsorption characterization (see Supporting Information Figure SI4). The surface area per gram for c-In2O3 is determined to be 91 m2/g, which is proportional to the nitrogen monolayer adsorbed, and the pore volume is 0.1135 cc/g using the BET model. A mesoporous value of 4.3 nm is determined using the BarrettJoynerHalenda (BJH) method pore size distribution model. UVvisible absorption studies also revealed a startling difference in the absorption patterns of h-In2O3 and c-In2O3. h-In2O3 prepared from sheetlike and spherical structure gave rise to a sharp absorption edge whereas the c-In2O3 prepared from the rodlike structure showed a broad peak from about 325 to 600 nm, almost covering the entire visible spectral region (see Supporting Information Figure SI5). This difference might arise from the large amount of surface state at the interface of the oriented particles, which could lead to a larger set of applications for the InKClA samples.

’ CONCLUSION In summary, we show that different assemblies of oriented nanorods and sheetlike morphologies can be prepared by carrying out the reaction in a strong electrolytic solution, which in effect changes the interfacial pressure between the growth region and the solution by controlling the monomer structure. The systematic analysis clearly shows that halide ions are not directly associated with the framework assembly of crystals (do not make a complex). The formation of cubic and hcp structures in the presence of chloride and iodide anions under identical conditions suggests that anions play a central role to control the crystalline structure by influencing the intermolecular interactions. In the presence of chloride ions and iodide ions, rodlike morphology and sheetlike morphologies are favorable, which upon being annealed lead to oriented mesoporous mesocrystalline c-In2O3 nanorod bunches and webbed sheetlike h-In2O3, respectively. The phase of the annealed material is decided by whether the nucleation takes place on the surface of indium hydroxide or at the defects in indium hydroxide, i.e., a single-crystalline In(OH)3 decomposes to h-In2O3 whereas polycrystalline In(OH)3 decomposes to c-In2O3. This scheme of growth could be extended to other inorganic systems because it manipulates the behavior of 9391


The Journal of Physical Chemistry C the interface. As the solution can be reused, this process is beneficial to the environment.

’ ASSOCIATED CONTENT

bS

Supporting Information. FESEM images, supporting experiments, and porosity and optical absorption measurement. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: 91-33-24734971. Fax: 91-3324732805.

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