Hierarchical In(OH)3 as a Precursor to Mesoporous In

Mar 15, 2014 - a period of time form well-defined In(OH)3 nanocubes. Mediation of poly(ethylene glycol) in optimal concentration is also found to be i...
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Hierarchical In(OH)3 as a Precursor to Mesoporous In2O3 Nanocubes: A Facile Synthesis Route, Mechanism of Self-Assembly, and Enhanced Sensing Response toward Hydrogen Arunkumar Shanmugasundaram,† Boppella Ramireddy,† Pratyay Basak,*,† Sunkara V. Manorama,*,† and Sanyadanam Srinath‡ †

Nanomaterials Laboratory, Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, Andhra Pradesh, India ‡ School of Physics, University of Hyderabad, Hyderabad, Andhra Pradesh, India S Supporting Information *

ABSTRACT: Mesoporous In2O3 nanocubes were achieved in this present work following a transformation from hierarchical structures of mesoporous In(OH)3 nanocubes synthesized hydrothermally. Appreciable control on the morphology of In(OH)3 nanostructures was attained by optimizing the reactant concentration, ratio of structure directing agent in the reaction medium (water/PEG ratio), the reaction temperature, and time. The synthesized samples were characterized extensively by XRD, TG-DTA, micro-Raman, and UV-DRS. Surface area and pore size distribution were determined from N2 adsorption and desorption isotherms. Morphological evaluations carried out using electron microscopy in scanning (FE-SEM) and transmission (TEM) mode not only provided information on the size and shape of the materials but also revealed the hierarchical assembly consisting of primary and secondary structures. Controlled studies as a function of various reaction parameters and the morphological evolution observed are rationally correlated to propose a plausible formation mechanism. Further, hydrogen gas sensing properties (sensitivity, sensor response, and recovery time) of the asprepared In2O3 nanostructures (nanocubes, nanobricks, nanoflakes, and nanoparticles) were investigated to demonstrate the influence of morphology. Owing to the porous structures and large surface area, In2O3 nanocubes exhibit superior sensitivity with short response/recovery times at concentrations as low as 100 ppb. Surface decoration with Pd nanoparticles activates these nanocubes, promoting excellent sensing response and selectivity toward hydrogen at room temperature.

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INTRODUCTION

Recent advances in synthesis strategies for the preparation of exotic nanostructures usually involve Ostwald ripening and/or oriented attachment self-assembly mechanisms. The liquidphase templating/ligand based approaches have received considerable attention because of the simple reaction conditions. The coordination of the metal salt and organic surfactants allows appreciable control on the crystal growth dictating the final morphology. Over the past decade several stimulating methods have been reported for the preparation of complex inorganic nanostructures, especially In2O3.34−39 Recently, Chen et al. reported In(OH)3 nanocubes with asparagine as an alkaline source,40 and Li et al. synthesized In(OH)3 hollow microspheres through the formamide− resorcinol−water system in which NH3 produced in situ on formamide hydrolysis acted as the base.37 Zai et al. reported the nearly monodispersed In(OH)3 hierarchical nanospheres and nanocubes, and they could successfully control the morphology by the ligand/In3+ ratio.41 Understanding the crystal growth

One of the most effective demonstrations to showcase the improved performance of nanostructured materials is chemical gas sensing1−3 which has a significant role to play in environmental monitoring/health sector.4−6 Wide band gap semiconductors such as SnO2,7,8 ZnO,9,10 WO3,11,12 Fe2O3,13 In2O3,14−17 etc., are now well documented as gas sensors. Of them, In2O3 has received considerable attention owing to its excellent optoelectronic properties18,19 as demonstrated in several modern electronic devices.20−25 Apart from its applications as electrodes, the material shows strong interaction with oxidizing15,26 and reducing gases27,28 which can be exploited to design ultrasensitive gas sensors. The past decade has seen tremendous efforts focused on developing In2O3-based gas sensors to improve sensitivity, selectivity, stability, and response/recovery times. Such attempts have employed diverse approaches such as doping,29−32 size reduction, 33 and morphology control.34,35 Many studies have reported that hierarchical structures increase both gas sensitivity and response speed, owing to the formation of porous structures.36,37 © 2014 American Chemical Society

Received: January 30, 2014 Revised: March 13, 2014 Published: March 15, 2014 6909

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Figure 1. Electron microscopy images of In(OH)3 nanocubes in scanning and transmission mode. Representative FE-SEM images of In(OH)3 nanocubes at (a) low and (b) high magnifications; corresponding TEM images of In(OH)3 nanocubes at (c) low and (b) high magnification; (e) selected area electron diffraction (SAED) pattern of the as-prepared In(OH)3 nanocubes and (f) powder X-ray diffraction pattern of In(OH)3 nanocubes.

particles). In particular, mesoporous In2O3 nanocubes exhibit superior sensitivity (SR ∼ 5%) even at very low concentrations (∼100 ppb), with appreciably fast response (ΓRes)/recovery (ΓRec) time. The excellent sensing performance of mesoporous nanocubes can be undoubtedly ascribed to the 3D-hierarchical structures of In2O3 with high surface area (∼234 m2/g). Significant improvement in sensor response at room temperature and selectivity could be further achieved with incorporation of Pd nanoparticles. Interestingly, the sensor demonstrated a strong preferential response toward hydrogen compared to other interfering gases such as ethanol, acetone, hexane, and ammonia. Plausible mechanisms for the formation of the hierarchical structures, the gas sensing, and the influence of Pd nanoparticles on enhanced hydrogen sensing have been proposed.

behavior and self-assemblies that form hierarchical structures with uniform morphology in wet-chemical synthetic routes necessitates a comprehensive effort to study the effect of the diverse parameters involved in the synthesis. Despite all these considerable efforts devoted to generate highly efficient 3D-hierarchical structures with exotic morphologies and recent successes in sensing applications, the demand, opportunities, and challenges leave room for further exploits. In this report, we present a facile synthesis route toward achieving hierarchical assembly of In2O3 nanostructures, evaluate the parameters that allow appreciable control on the reaction, discuss the formation mechanism, and demonstrate the enhanced sensing response toward hydrogen. The synthesis entails preparation of the intermediate In(OH)3 nanostructures under hydrothermal conditions using InCl3 as a precursor, a mixed solvent system of water and poly(ethylene glycol) (water/PEG) as the reaction medium, and ethanolamine as an organic base. Several different morphologies of In(OH)3 nanostructures, such as nanocubes, nanobricks, nanoflakes, and nanoparticles, could be successfully achieved through simple reaction parameter modulations, i.e., water/PEG ratio. Subsequent calcination at 400 °C transformed these In(OH)3 nanostructures into In2 O 3 while retaining the similar morphologies and sizes, with enhanced porosities. Comprehensive characterizations along with detailed supporting electron microscopy studies reveal the formation of two phases of In(OH)3 and In2O3 subsequently. Correlation of reaction parameters and morphological evaluations using electron microscopy provided a thorough understanding on the formation mechanism of hierarchical nanostructures. The enhanced activity and gas response behavior were evaluated by studying the resistance changes of these materials against hydrogen. Detailed sensing studies revealed that the sensor response is greatly influenced by the morphology of In2O3 nanostructures (nanocubes, nanobricks, nanoflakes, and nano-

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EXPERIMENTAL SECTION Chemicals. Indium chloride (InCl3), palladium chloride (PdCl2), sodium borohydride (NaBH4), and poly(ethylene glycol) (PEG, Mn ∼ 400) were purchased from Sigma-Aldrich, and ethanolamine (NH2CH2CH2OH) was purchased from Sd Fine Chemicals Ltd. All chemicals are A.R. grade and used without further purification. Millipore deionized water (18.2 MΩ at RT) was used throughout the experiment. Preparation of In(OH)3 Nanostructures. In a typical synthesis procedure, ca. 0.22 g (1 mmol) of indium chloride was dissolved into 10 mL of water under vigorous magnetic stirring. To this solution, 2 mL of ethanolamine was added slowly, followed by addition of 10 mL of poly(ethylene glycol) (PEG, Mn = 400) after 15 min under stirring conditions. The solution was transferred into a Teflon-lined stainless steel autoclave after another 15 min and placed in a programmable oven. The temperature was raised to 220 °C at a heating rate of 10 °C/min and held at constant temperature for 24 h. After the completion of reaction, the resultant mixture was centrifuged to 6910

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Figure 2. Representative FE-SEM images of the as-synthesized product obtained at 220 °C with volume ratio of 1:0.2:1 of water:ethanolamine:poly(ethylene glycol) in the reaction medium for different reaction times (a) 1.5, (b) 3, (c) 6, (d) 12, and (e) 24 h for the formation process of In(OH)3 nanocubes and (f) corresponding powder X-ray diffraction pattern.

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obtain a white product. Repeated washing with deionized water ensured complete removal of any residual impurities. Finally, the product was dried in an oven at 60 °C and the crystal structure confirmed by XRD. Preparation of In2O3 Nanostructures. The presynthesized white In(OH)3 precursors were placed in a quartz crucible and calcined at 400 °C for 3 h in a conventional furnace under ambient atmosphere and pressure. The heating rate for the oven was maintained at 10 °C/min. After 3 h, the samples were oven cooled to obtain a pale yellow powder which was confirmed by XRD to be In2O3. Incorporation of Palladium Nanoparticles. Synthesis of In(OH)3 and its transformation to In2O3 were achieved following the methods described in the preceding section. To incorporate different weight percentages of palladium, a solution impregnation technique was employed. The required amount of palladium chloride (PdCl2) salt was added to the as synthesized powder suspended in an aqueous medium under magnetic agitation. Thereafter, sodium borohydride (NaBH4) was used to reduce the palladium salt, Pd(II), to metallic palladium, Pd(0), in situ. Gas Sensor Fabrication and Measurements. The schematic of the sensor assembly, techniques used for fabrication, and sensor characteristics measurement setup have been described extensively in our previous works42,43 and described briefly in the Supporting Information. The gas sensing measurements were carried out by monitoring the changes in resistance under a constant applied voltage during cyclic exposure to different gas concentrations. Sensor response is defined as S = Ra/Rg, where Ra is the sensor resistance in air and Rg is the sensor resistance in the presence of gas. Response time (ΓRes) of the sensor is defined as the time taken by the sensor to reach 90% of its saturation limit after exposure to test gas, while the recovery time (ΓREC) is defined as the time taken for the sensor to reach 10% of its original resistance value once the target gas is switched off.

RESULTS AND DISCUSSION

Morphological and Crystal Structure Evaluations. Morphological evaluations of the as-prepared In(OH)3 nanocubes were assessed by electron microscopy in both scanning and transmission mode. Figure 1a is a typical low-magnification FE-SEM image showing numerous cubes nearly uniform in size with an average diameter of 200 nm. At higher magnification (Figure 1b) the morphologies reveal clearly defined, wellfaceted, and sharp edges of the nanocubes. These as-prepared In(OH)3 nanocubes were further characterized by TEM, and as represented the dimensions and size distributions (Figure 1c) concurred well with the FE-SEM studies. Transmission microscopy at higher magnifications, however, reveals a contrast based on the electron-dense and electron-poor regions within the nanocube structures, giving an impression of an apparently porous morphology (Figure 1d). Selected area electron diffraction patterns show Laue spots along with the rings, indicating considerable crystallinity of the synthesized material (Figure 1e). The SAED could be indexed to the cubic phase of In(OH)3, where the primary reflection is found to be from the (200) plane, which derives from {100} family of planes of these nanocrystals.41,44 The other diffraction rings corresponding to the planes (220), (222), and (400) as assigned confirm the formation of In(OH)3 nanocubes. Concurrently, the powder X-ray analysis performed also conforms to the crystal structure and phase purity of the products. The well-resolved Bragg’s diffraction (Figure 1f) could be perfectly indexed to the bcc-In(OH)3 crystal structure with an Ia3 (204) space group.45 Sharp and strong diffraction peaks signifies the high crystalline quality of the as-prepared products. The calculated lattice constants a = b = c = 0.797 nm are in good agreement with the standard reported value (JCPDS Card No. 85-1338, a = 0.7979 nm).46 The diffraction pattern of In(OH)3 also reaffirmed that the major diffraction peak at 2θ = 22.27° corresponds to the (200) plane. The observations suggest that the growth of In(OH)3 nanocubes 6911

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Figure 3. FE-SEM images of the as-prepared In(OH)3 products obtained at 220 °C with different water/PEG ratio of (a) 1:0, (b) 3:1, (c) 1:1, (d) 1:3, and (e) premixed 1:1 water/PEG and (f) corresponding powder X-ray diffraction pattern.

takes place preferentially along the ⟨100⟩ directions.41 Absence of diffraction peaks other than bcc crystal structured In(OH)3 indicates the phase purity of the samples. A series of controlled experiments were carried out to comprehensively understand the formation process and underlying mechanism of In(OH)3 nanocubes. The extensive role and effect of these parameter variations are discussed in the following sections. Effect of Reaction Time. In general, the reaction kinetics, nucleation/growth, and ripening have a profound effect on the final morphology of the products and is exclusively timedependent. The influence of reaction time on the formation of In(OH)3 nanocubes was hence investigated as a variable while keeping all other reaction parameters the same. A volume ratio for the medium, i.e., water:ethanolamine:PEG, was maintained as 1:0.2:1 throughout this study. The morphological evolution of In(OH)3 nanocubes obtained at different reaction stages are depicted through a series of FE-SEM images provided in Figure 2a−e. In the initial stages, as exemplified for 1.5 h interval (Figure 2a), the isolated products show typically clustered aggregates of smaller nanoparticles with an average size ∼10 nm. When the reaction time is extended to 3 and 6 h (Figure 2b,c), indications of semiformed nanocubes appear interspersed among the aggregated smaller nanoparticles. For reaction times of 12 h and beyond, (Figure 2d,e) a clear indication on the formation of In(OH)3 nanocubes is evident. The size distributions of irregular-shaped particles are however quite broad, consisting of both large and small nanocubes. When the reaction time is prolonged further to 24 h and beyond, nearly uniform sized nanocubes are obtained. The stacked XRD (Figure 2f) corresponding to each sample also indicates the increased crystallinity in the samples. It is clearly evidenced from the time-dependent studies that the initial formation of small aggregated nanoparticles within the medium undergoes slow transformation into nanocubes via either a dissolutionoriented reattachment mechanism or directed self-assembly. As aging process continues, the smaller nanocubes grow into bigger nanocubes via the Ostwald ripening process.

Effect of Reaction Medium. The reaction medium, especially in mixed solvent approaches, plays an important part in wet chemical routes. To investigate, the constituent ratios of water and PEG in the solvent system were varied while keeping all other parameters constant. Poly(ethylene glycol) has been popularly used as reducing and structure directing agent. A series of experiments were hence carried out varying the water/PEG ratio to determine the role of poly(ethylene glycol) in the formation of these In(OH)3 nanocubes. As a control experiment, when only water is used as a reaction medium, the resultant products are irregularly shaped nanoflakes, as in Figure 3a. When a volume ratio of 3:1 is maintained for water/PEG (typically 15 mL:5 mL) in the reaction medium, the sample characteristically shows a bricklike morphology with a compact z-axis (Figure 3b, size ∼200 nm × 200 nm × 50 nm). At a condition where the water to PEG volume ratio is equal (1:1) (typically 10 mL:10 mL), regular formation of In(OH)3 nanocubes (∼200 nm × 200 nm × 200 nm) is observed as depicted in Figure 3c. When the volume of water is significantly reduced, as in the case of Figure 3d, water/PEG = 1:3 (5 mL:15 mL), the final product invariably showed cluster of smaller irregularly shaped nanoparticles with sizes ∼10−20 nm. Our findings also revealed that the sequence of reagent addition also plays an important part in the product morphology. Interestingly, these observations strongly indicate possibilities of intermediates, stabilized complexation, and/or ligand exchange processes that are probably coupled together. To exemplify, in the preceding reactions discussed, the poly(ethylene glycol) was added to the reactant mixture in the final stages followed by autoclaving. For a change, when the precursors are added to a solvent mix with poly(ethylene glycol) already present in the volume ratio 1:1, the morphology of the synthesized In(OH)3 nanocubes samples show a remarkable change apart from being smaller in size (∼20−30 nm). As evidenced in Figure 3e, the surface of these nanocubes are quite rough, and closer observations reveal that each of these nanocubes are actually composed of numerous ultrafine 6912

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Scheme 1. Representative Illustration Summing Up the Effect of Different Synthesis Parameters on the Final Morphology of the In(OH)3 Nanostructures

nanosquares (Figure S4a,b) formed at 140 °C, transformed into larger irregular shaped and thicker bricklike (Figure S4c,d) assemblies at 180 °C, and finally nearly uniform shape and sized nanocubes were obtained when the reaction was carried out 220 °C for 24 h. Formation Mechanism and Growth of In(OH)3. As discussed in the preceding sections, on the grounds of the above observations coupled with recent literature reports on similar attempts from several researchers,40,47 a plausible mechanism on the formation and growth of the In(OH)3 nanostructures can be proposed. The synthesis of In(OH)3 under the hydrothermal reaction conditions using InCl3 as the metal-ion precursor, ethanolamine as the organic base, and poly(ethylene glycol) as the structure directing agent in the present study can be simply expressed as In3+ + 3H2O → In(OH)3 + 3H+. The hydrolysis and nucleation are catalyzed by the basic ethanolamine. At the same time, as the Lewis base, ethanolamine can possibly coordinate with the In3+ because of coordination effects.48,49 With the increase of reaction temperature, the ethanolamine begins to hydrolyze to give off ammonia in situ, leading to a uniform rise in the pH value of the solution. As proposed in earlier reports, this can effectively arrest the local supersaturation events and favor homogeneous nucleation of tiny single crystals from the complex precursor with appreciable degree of control.50−54 The presence of ethanolamine, PEG, and the Cl− ions generated in situ in the solutions helps to control the nucleation and stabilize the In(OH)3 seed. This is followed by the growth of nuclei consuming solute molecules via a diffusive mechanism to form crystalline subunits. At the initial stage of reaction, the nuclei and growth proceeded slowly, to yield numerous 0D In(OH)3 nanoparticles. Once the small particles are formed in the reaction solution, they are active and proceed to selfaggregate to reduce their surface energies. Under the present

smaller nanoparticles of size ∼5 nm. That the degree of crystallinity achieved is higher for the volume ratio 1:1 of water/PEG and least for the same when the stage of precursor addition is reversed is apparent from the XRD provided in Figure 3f. The observations lead us to infer the determining role of poly(ethylene glycol) as both a structure directing and stabilizing agent. Effect of Ethanolamine. The influence of ethanolamine, a popular Lewis base used for controlled hydrolysis as well as a stabilizing agent during crystal growth, was also investigated in the process. A series of controlled experiments were carried out with varying concentrations of ethanolamine to obtain the In(OH)3 nanostructures. The samples were characterized in detail by FE-SEM and XRD as well, and the images/diffraction patterns are provided as Supporting Information (Figures S2a− d and S3). The noticeable role played by the reagent on determining the final morphologies is unmistakable as observed in FE-SEM and XRD. With increasing concentration of ethanolamine with respect to water, i.e., 0.2, 0.6, and 1.0, we observe a progressive stabilization of the primary particles formed, leading to an ultrafine product (sizes ∼5−10 nm). Close observation of the sample with ethanolamine volume ratio = 0.6 reveals larger nanocube-like partial formations of these primary particles (Figure S2b). At higher concentration (1.0 v/v), only ultrafine particles (Figure S2d) with appreciable monodispersity is clearly observed. Effect of Temperature. Hydrothermal reaction condition is an equally critical variable, and hence the effect of reaction temperature is also investigated. The temperature dependence of product morphology was also analyzed in considerable detail and provided as Supporting Information (Figure S4a−d). The consequence of increasing temperature on degree of crystallinity and ordered self-assemblies of primary In(OH)3 nanoparticles is very apparent. The transparent thin platelike 6913

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nanoparticles can become dominant due to high rate of nucleation under higher base concentration. This is very evident from our observation when high concentration of ethanolamine is used while holding all the other parameters constant. Apart from our observations regarding the oriented attachment and face inhibition effects, reaction temperature is also found to play a key role. All other conditions kept similar, at lower temperatures (i.e., 140 °C) the growth rate is preferably more in the [100] directions, leading to thin and transparent nanosquares. With increasing temperature, it is evident that the growth rates in the [110] and [111] directions contributed significantly to the final morphology. The nanosquares transformed into nanobricks, and finally when the growth rates for [100] to [111] is almost equal to half (0.58), the morphologies obtained were almost uniform nanocubes. Phase Transformations of In(OH)3 to In2O3. Indium hydroxide can be easily transformed to its corresponding oxide form on thermal treatment (Scheme 2). The nanostructured

experimental conditions, primary nanoparticles/subunits thus form large assemblages to minimize their surface energy through an oriented attachment mechanism aided by the poly(ethylene glycol), ethanolamine, and chloride ions. Owing to the alternately hydrophilic and hydrophobic structures in its polymeric chains, the PEG behaves similar to a nonionic surfactant and aggregates could possibly form in water that can act as templates.55−58 The studies demonstrate that these nanoparticles just encapsulate the PEG aggregates inside and form a crystalline cubical shell. That the oriented attachment mechanism is coupled with Ostwald ripening is quite evident from the time-dependent electron microscopy studies, where the primary particles self-assemble into cubelike forms and over a period of time form well-defined In(OH)3 nanocubes. Mediation of poly(ethylene glycol) in optimal concentration is also found to be important in determining the final morphology as represented schematically (Scheme 1). Preferential adsorption of molecules and ions in solution to different crystal faces directs the growth of nuclei into various shapes was pointed out by Murphy et al.59 Appropriate control on the growth rates along different crystal axes can thus effectively allow to manipulate the final morphology. For the crystal shape to reflect its cubic geometry, Wang60 proposed that the ratio of the growth rate (R) in [100] to that in [111] directions is critical and plays an important role in determining the final morphology of a crystal with cubic lattice. It has been proposed that when the rate of growth is equal for a cubic lattice, R = 1, the geometrical shape is cuboctahedral in nature. When the ratio R is appreciably low (∼0.58), i.e. the rate of growth in ⟨111⟩ direction is higher, the addition of planes {111} occurs rapidly to transform into a perfect cubic geometry in due course. Thus, once the corners are formed, it can probably be understood that these particular planes disappear (i.e., the ⟨111⟩ plane/s with the fastest growth rate will disappear). The rationale can be logically extended in the present case where the In(OH)3 cubes bound by the six {200} planes are formed. In the absence of PEG, the face inhibitive function is being provided by only ethanolamine and Cl− ions (smaller molecules/ions) and possibly anisotropic in nature. Taking a cue from catalysis for these cuboctahedral seeds, the {111} and {110} planes probably provides for the most active surfaces for adsorption of ethanolamine and Cl− ions leading to surface stabilization. This would considerably restrict the growth along the ⟨111⟩ and ⟨110⟩ directions, and under these circumstances, the [100] growth direction is the preferred one leading to initial formation of rodlike structures. Eventually these rods align/selfassemble to minimize surface energies, giving an appearance of nanoplate/nanoflake-like structures. A similar argument in presence and absence of surface stabilizing agents was extended by Huang et al.,47 wherein they observe preferred growth patterns in ⟨100⟩ directions coupled with minor contribution in ⟨110⟩ for In(OH)3 to form nanorods which undergo oriented attachment and assemble to transform into nanosheets. By introducing PEG in the reaction system, the growthinhibited faces disappeared and anisotropic growth probability might be weakened greatly owing to competitive stabilization. The effect is clearly observed with increasing concentration of PEG, where the platelike morphology increasingly transformed to bricklike shapes and finally to well-defined cubes. However, at excessively high concentration of PEG the primary nanoparticles are probably stabilized as such and fail to selfassemble. Similarly, the formation and aggregation of primary

Scheme 2. Pictorial Representation of the Formation of In(OH)3 Nanocubes Followed by Transformation to In2O3

In(OH)3 synthesized in various morphologies were calcined at different temperatures to investigate the phase change, thermal stability, and structural integrity. Phase transformation of In(OH)3 into In2O3 was analyzed using thermogravimetry from room temperature to 800 °C at a scan rate of 10 °C/min under a nitrogen atmosphere. A representative thermogravimetric profile (Figure 4A) of In(OH)3 nanocubes reveals three stages of weight loss. Preliminary weight loss observed between RT and ∼200 °C can be attributed to the removal of free and bound water/OH from the surface of In(OH)3 nanocubes. The second stage of significant weight loss observed from 200 to 280 °C corresponds to the degradation onset of encapsulated poly(ethylene glycol) and ethanolamine from the system. The third and the final weight loss occurred at 280−450 °C and can be attributed to the chemical dehydration of all the residual hydroxide (2In(OH)3 → In2O3 + 3H2O) completing the formation of In2O3 phase.42 Throughout this phase transformation process, nearly ∼20% of weight loss has been observed consistently, which is in good agreement with earlier reports.61 All other In(OH)3 nanostructures prepared in this work present similar thermogravimetric (Figure S6) profiles. Corresponding crystal structure as a function of calcination temperature was also investigated in parallel. The as-prepared materials were calcined at different temperatures from 200 to 600 °C for 4 h in a conventional oven. Figure 4B shows the sequence powder X-ray diffraction patterns of (a) assynthesized In(OH)3 nanocubes, which correlate and can be perfectly indexed to the bcc crystal structured In(OH)3 nanocubes, (b−d) after subsequent calcination at 200, 400, and 600 °C. At and above 400 °C, major diffraction peaks (2θ =22.27° (200)) of as-prepared In(OH)3 nanocubes disappeared, and the corresponding patterns could be indexed to bcc crystal structure of In2O3with the space group Ia3 (206).45 6914

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Figure 5. (a−c) FE-SEM images of In2O3 nanocubes at different magnification. (d) TEM image of In2O3 nanocubes. Inset: SAED pattern of In2O3 nanocubes.

shows the corresponding electron micrographs of In2O3 nanocubes obtained from the In(OH)3 nanocubes subjected at 400 °C for 4 h in the conventional oven, at different magnifications. The findings reveal that the In2O3 nanocubes appreciably retain their original shapes even during phase transformation. Nevertheless, at temperatures ∼400 °C and beyond, with dehydration and capping agents (PEG, EA) burnoff, significant surface roughness and porosity can be observed on the nanocube surfaces. Indications of slight structural instability for these nanocube assemblages are also visible with appearance of smaller cuboids and damaged edges. These inferences are further supported by the transmission microscopy studies as presented in Figure 5d, where the inter connected In2O3 nanoparticles displays high degree of mesoporosity. The selected area electron diffraction (SAED) patterns (inset: Figure 5d) could be indexed to the (200), (400), (431), and (400) planes of the bcc crystal lattice−In2O3. Similar findings on the other morphologies of In(OH)3 are provided as Supporting Information (Figures S9a−d and S10). Optical properties of the as-prepared In2O3 nanocubes were evaluated using ultraviolet−diffuse reflectance spectroscopy (UV-DRS), recorded in the reflectance mode (%R) (Figure S11). A reflectance band edge of In2O3 nanocubes was observed in the range of 350−450 nm. The optical band gap energy (Eg) for direct and/or indirect band gap materials is related to the linear absorption coefficient (α) near the absorption edge which is expressed as αhv = C1(hv − Eg)1/2 and αhv = C2(hv − Eg)2, where hv is the photon energy, α is the absorption coefficient, and C1 and C2 are the corresponding constants for the material, respectively.71 The absorption edge is commonly fit to direct allowed transition where n = 1/2 and the indirect allowed transition with n = 2.72,73 The inset in Figure S11 depicts the Kubelka−Munk transformed reflectance spectra for both the direct and indirect cases of transitions. The optical band gap (Eg) from the [F(R∞)hv]2 and [F(R∞)hv]1/2 versus photon energy (hv) plots estimated for the nanocubes are 3.56 and 2.72 eV, which matches excellently with the

Figure 4. Thermal stability studies: (A) thermogravimetric−differential thermal analysis of as-prepared In(OH)3 nanocubes (B) powder X-ray diffraction pattern of (a) as-prepared In(OH)3 nanocubes and after calcined at different temperatures: (b) 200, (c) 400, and (d) 600 °C.

The calculated lattice constants of a = b = c = 1.011 nm is in good agreement with the standard value reported.62 Absence of any other peaks beyond 400 °C indicated no further phase transformation of In2O3. Micro-Raman analysis of the as-prepared material provides additional inputs on the vibrational modes, sample homogeneity, and phase confirmation. The stacked Raman spectra collected at room temperature are presented in Figure S7. Characteristic Raman modes of In(OH)3 centered at 208, 309, 356, and 391 cm−1 in conformity to the reported peak positions can be observed in Figure S7a.63,64 As also supported by XRD, the Raman spectra of the calcined samples at 400 °C (Figure S7b) show only the characteristic peaks of In2O3. Group theory analysis predicts the (4Ag + 4Eg + 14Tg + 5Au + 5Eu + 16Tu) vibrational modes for In2O3. The symmetric vibrational modes Ag, Eg, and Tg are Raman-active but infrared-inactive modes. While Au and Eu both are infrared- and Raman-inactive vibrational modes, the Tu mode is Raman-inactive and infraredactive mode.65,66 In the present study, only six phonon modes pertaining to E1g (110, 133, 301 cm−1), E2g (360, 624 cm−1), A1g (490 cm−1) associated with the bcc-structured In2O3 could be distinctly detected.65,67 The Raman bands observed at 133 cm−1 corresponds to In−O vibrations48 while the 490 and 631 cm−1 are usually interpreted as the vstretching modes of InO6 octahedrons.68 The Raman shift observed at 301 cm−1 is assigned to bending (δ) vibration of InO6 octahedra structural units, and the 360 cm−1 band is associated with the stretching vibration mode of In−O−In plane.69,70 Detailed morphological evaluations were also carried out postcalcination to ascertain the structural integrity and/or changes that occur upon temperature treatment. Figure 5a−c 6915

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response of S ∼ 980. The sensor responses of nanobricks, nanoparticles, and nanoflakes are ∼500, ∼250, and ∼200, respectively, though lower are also definitely significant. Arrhenius plots of the sensor response, ln(S) vs Ts (sensing temperature), clearly indicates the strong linear dependence in the sensor response as a function of operating temperature (inset: Figure 6). All further gas sensing studies were carried out at the optimum working temperature of 150 °C to investigate the maximum sensitivity and dynamic response profiles of each sensor element fabricated using the four available morphologies. Figure 7 illustrates the dynamic sensor response of the four sensors fabricated using (a) In2O3 nanocubes, (b) nanobricks, (c) nanoparticles, and (d) nanoflakes toward different H2 concentration (5, 2, 1, 0.5, 0.2, and 0.1 ppm) at 150 °C. As expected, the sensor response varies linearly with concentration of hydrogen (shown as inset). The sensor elements fabricated using nanoparticles and nanoflakes apparently cannot show any response to hydrogen below 0.5 ppm. The H2 response− recovery times (ΓRes − ΓRec) of all four sensors elements are presented in Figure 8. The response and recovery time of each sensor toward 5 ppm H2 are estimated to be 67, 36, 39, 33 s and 143, 100, 115, 92 s, respectively. Though the studies unambiguously demonstrate that all the sensing elements possess good capability for the detection of H2, the sensor fabricated using the mesoporous nanocubes exhibited much higher response and linearity over a wider concentration range (sensitivity) (from 5 ppm; S ∼ 26 to 0.1 ppm; S ∼ 3.2), indicating its excellent sensor response. The above investigations directly imply that the magnitude of sensor response is greatly influenced by the morphology of In2O3 nanostructures, and the sensing performance of nanocubes is far superior to the other morphologies. To improve the sensor response further and achieve higher efficiencies at ambient temperature, the nanocubes were decorated with palladium nanoparticles (Pd-NPs) in situ. Sensor response of Pd-NPs loaded In2O3 nanocubes to H2 gas was studied as a function of different operating temperatures and provided in Figure 9a. A loading of only 0.25 wt % Pd-NPs shows an enhanced sensing response of (S) ∼50 at 35 °C, and this reaches the maximum sensor response (S) ∼3500 at 150 °C. A slight increase in loading concentration (0.50 wt %) of Pd-NPS shows excellent sensing response of (S) ∼280 at 35 °C and maximum sensor response of (S) ∼9000 at 150 °C. The bar plot (Figure 9b) clearly shows a comparative sensor response of 0.25 and 0.5 wt % Pd-NPs loaded In2O3 nanocubes at room temperature (∼35 °C) and at 50 °C. Time dependence of the sensor response for a 0.5 wt % Pd-NPs loaded nanocubes to various H2 concentrations (500 and 1000 ppm) at 35 °C are shown in Figure 9c. In the presence of 1000 ppm H2 gas the sensor response gradually increases and attains saturation ∼20, with the response (ΓRes) and recovery time (ΓRec) of about 117 s and 210 s. For any practical gas sensor, selectivity in the presence of other interfering gases is an important factor that also needs to be considered. Hence, the selectivity of (0.5 wt %) Pd-NPs loaded In2O3 was tested against other interfering gases such as ethanol, ammonia, hexane, acetone, and hydrogen taken as 1000 ppm in normal air at 35 °C, presented in Figure 9d. The sensor response to hydrogen is ∼4 times higher than the sensor response to other interfering gases indicating its high selectivity toward H2. Sensing Mechanism. Enhanced sensing performance of In2O3 nanocubes could be postulated on the following lines.

reported values (direct band gap 3.55−3.75 eV and indirect band gap 2.6−2.7 eV) for In2O3.39,74,75 Surface area and pore size distribution of as-prepared In2O3 nanocubes were measured by nitrogen adsorption−desorption isotherms and BJH (Barrett−Joyner−Halenda) pore size distribution analysis. The adsorption−desorption profile (Figure S12) exhibits type IV isotherms with hysteresis loop at high relative pressure. The corresponding pore size distribution plots (inset in Figure S12) indicates the mesoporous nature of the samples. Analysis reveals that these mesoporous In2O3 nanocubes have numerous pores in the range of 4−16 nm. The cumulative pore volume for In2O3 nanocubes estimated from the BJH technique is ∼1.080 cm3/g. The corresponding BET specific surface area of In 2 O3 nanocubes calculated as 234 m2/g was appreciably high. Gas Sensing Studies. Hydrogen has competitively positioned itself as the next-generation clean energy fuel for modern industries and transportation sector.76−78 However, a LEL (lower explosive limit) of 4 vol % coupled with its colorless and odorless nature poses a challenge for safety requirements and precautions.79,80 An effective sensor for early detection of leaks and quantification at low concentration and at ambient temperature can help mitigate risks and is crucial for the industries where safety is a major concern. Hydrogen sensing capability of In2O3 nanocubes, nanobricks, nanoflakes, and nanoparticles were carried out at different sensing temperatures (Ts) in a dry air environment. It is well understood that a sensor would have maximum sensitivity at the operating temperature where the temperature-dependent, adsorption−desorption kinetic process is optimum.81 To determine the optimum sensing temperature, each sensor element fabricated was also individually tested as a function of different operating temperatures. Figure 6 shows the hydrogen

Figure 6. Sensor responses of In2O3 nanostructures of different morphologies (nanocubes, nanobricks, nanoparticles, and nanoflakes) as a function of different operating temperatures toward 1% hydrogen gas. Inset: typical Arrhenius plot depicting the sensor response.

sensing (S) characteristics of nanocubes, nanobricks, nanoflakes, and nanoparticles toward 1% hydrogen gas as a function of different sensing temperatures. All sensors show the typical n-type sensing behavior, i.e., decrease in sensor resistance in H2 gas (reducing) environment. As observed from the plot (Figure 6), the sensor response gradually increases with increase in operating temperature, and the results clearly illustrate that the In2O3 nanocubes perform the best in terms of sensor response against all other morphologies studied. At an optimum working temperature (∼150 °C), encouragingly In2O3 nanocubes sensor exhibits a 6916

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Figure 7. Dynamic sensor response of In2O3 nanostructures (a) nanocubes, (b) nanobricks, (c) nanoparticles, and (d) nanoflakes as a function of different gas concentration at operating temperature of 150 °C [Rs: sensor response (Ra/Rg); Gc: gas concentration (ppm); Ts: sensing temperature (°C); ↑ indicates gas on and ↓ off].

particles with their inherent affinity toward hydrogen provide better adsorption and dissociation of hydrogen molecules, thus increasing the H2 to 2H+ conversion rates that would react with the adsorbed oxygen species, resulting in a back-transfer of electrons to the In2O3 surface leading to a rapid increase in the conductivity leading to higher sensing performance. In another proposition, it has been extended that catalytic properties of palladium nanoparticles apart from aiding hydrogen adsorption and dissociation also help dissociate molecular oxygen to generate the oxygen species. This is then spilled onto the oxide surface (even at room temperature)82,83 by “chemical sensitization”. This explains why the optimum working temperature decreases with increasing amount of palladium. The modulation of work function is attributed to the contacts between palladium nanoparticles and metal oxide surfaces that leads to the formation of Schottky barriers or depletion region (difference in work function of semiconductor/metal nanoparticles), which alters the device conductance,4,84 and the depletion region around palladium nanoparticles/semiconductor enhancing sensor response for the gas detection. All these factors coupled with the appreciable porosity of as-synthesized In2O3 nanocubes support its excellent gas sensing response. The channels of 3D cocontinuous pores facilitate rapid gas diffusion and favor the adsorption−desorption kinetics, leading to a fast response toward the test gas.

Figure 8. Bar plot depicting sensor response and recovery time toward 5 ppm of hydrogen gas for the various morphologies studied.

Sensing mechanism mainly follows the gas adsorption−charge transfer−and desorption pathway. It could be established in the preceding sections that the mesoporous nanocubes composed of ultrasmall and oriented nanoparticles possess high surface area yet a 3D cocontinuity. The high surface to volume ratio coupled with the oxygen deficiency thus enables the sensing material surface more active sites for adsorption of molecular oxygen. The adsorbed oxygen on the sensor surface gets ionized by extracting the electrons from the conduction band of In2O3, creating chemisorbed oxygen species (O2−, O−, O2−), and these species are responsible for the altered electrical transport properties in reducing atmosphere which decide the sensor performance. The sensor response is additionally influenced by the incorporation of palladium nanoparticles on the oxide surface. This could be explained considering two rationale: (i) spillover effect and (ii) work-function modulation. Palladium nano-

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CONCLUSIONS In summary, we have demonstrated a facile synthesis strategy to obtain hierarchical structures of mesoporous In(OH)3 hydrothermally. Thermal transformation of these nanostructures to In2O3 postsynthesis were also achieved successfully while retaining their original morphology. It could also be established that an appreciable control on the morphology and selfassembly of In(OH)3 nanostructures can be exercised by 6917

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Figure 9. (a) Sensor response of In2O3 nanocubes with and without palladium loading as a function of temperature toward 1% hydrogen gas. (b) Bar plot illustrates the sensor response of palladium-loaded In2O3 nanocubes at RT (35 °C) and (50 °C). (c) Time-dependent dynamic sensor response of 0.5 wt % palladium-loaded In2O3 nanocubes toward 500 and 1000 ppm. (d) Bar plot depicts sensor response and selectivity of 0.5 wt % palladium-loaded In2O3 nanocubes toward 1000 ppm of ethanol, ammonia, hexane, acetone, and hydrogen at 35 °C.

higher than the sensor response to other interfering gases indicating high selectivity toward hydrogen.

manipulating the reactant concentration, ratio of structure directing agent in the reaction medium (water/PEG ratio), the reaction temperature, and time. Extensive studies employing electron microscopy revealed the hierarchical assembly consisting of primary and secondary structures and provided significant clues on the different stages of formation involved. Controlled studies as a function of various reaction parameters and the morphological evolution (nanocubes, nanobricks, nanoflakes, and nanoparticles) observed could be rationally correlated with the nucleation, growth, oriented attachment, and Ostwald ripening to propose a plausible formation mechanism. The findings from XRD, TGA, micro-Raman, and UV-DRS analysis were consistent with each other. Estimation of surface area and pore size distribution of the as-prepared nanostructures exhibit considerably high surface area with ∼3−5 nm pores. The influences of In2O3 morphology on the gas sensing properties were investigated in detail by evaluating the sensitivity, sensor response, and recovery time with respect to hydrogen concentration. At an optimum working temperature (∼150 °C), encouragingly In2O3 nanocubes sensor exhibits a response of S ∼ 980. The sensor response of nanobricks, nanoparticles, and nanoflakes are ∼500, ∼250, and ∼200, respectively, though lower are also definitely significant. The mesoporous nanocubes exhibited much higher response and linearity over a wider concentration range (sensitivity) (from 5 ppm; S ∼ 26 to 0.1 ppm; S ∼ 3.2), indicating its excellent sensor response. Owing to the presence of 3D cocontinuous pores that facilitates rapid gas diffusion and faster adsorption−desorption kinetics, In2O3 nanocubes exhibit superior sensitivity with short response/recovery times at concentrations as low as 100 ppb. Surface decoration with only 0.5% Pd nanoparticles activates these nanocubes promoting excellent sensing response at room temperature and is ∼4 times

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ASSOCIATED CONTENT

* Supporting Information S

Details of experimental methods and characterization techniques; gas sensor fabrication and measurements; FE-SEM images of the reactant mixture before hydrothermal reaction (Figure S1); effect of ethanolamine (Figure S2) on the preparation of In(OH)3 nanostructures and the corresponding powder X-ray diffraction pattern (Figure S3); influence of reaction temperature (Figure S4) and the corresponding powder X-ray diffraction pattern (Figure S5); TG-DTA of as prepared In(OH)3 nanostructures (Figure S6); micro-Raman spectra of as-synthesized In(OH)3 nanocubes (Figure S7a) and In(OH)3 nanocubes calcined at 400 °C (Figure S7b); microRaman spectroscopy analysis of as-prepared In2O3 nanobricks, nanoflakes, and nanoparticles (Figure S8); FE-SEM images of as-prepared In2O3 nanostructures: nanoflakes, nanobricks, nanocubes, nanoparticles (Figure S9); powder X-ray diffraction pattern of as-prepared In2O3 (a) nanoflakes, (b) nanobricks, (c) nanocubes, and (d) nanoparticles (Figure S10); diffuse reflectance UV−vis spectra of the hierarchical mesoporous particle assembled In 2 O 3 nanocubes (Figure S11); N 2 adsorption−desorption isotherms and corresponding BJH pore size distributions of In2O3 nanocubes (Figure S12); N2 adsorption−desorption isotherms and corresponding BJH pore size distributions of In2O3 nanobricks, nanoflakes, and nanoparticles (Figure S13); BET surface area analysis of In2O3 nanobricks, nanoflakes, and nanoparticles (Figure S14a− d); elemental analysis of as-prepared In2O3 nanocubes, 0.25 wt % loaded In2O3 nanocubes, and 0.5 wt % loaded In2O3 nanocubes (Figure S15a−c). This material is available free of charge via the Internet at http://pubs.acs.org. 6918

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AUTHOR INFORMATION

Corresponding Authors

*Tel +91-40-27193225, Fax +91-40-27160921; e-mail [email protected] (P.B.). *Tel +91-40-27193225, Fax +91-40-27160921; e-mail [email protected] (S.V.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Arunkumar Shanmugasundaram, Boppella Ramireddy, acknowledges the Council of Scientific and Industrial Research (CSIR), University Grant Commission (UGC) India, for the Senior Research Fellowship (SRF). P.B. and S.V.M. duly acknowledge the strong support of MNRE-CSIR TAPSUN Project on Dye Sensitized Solar Cells (DyeCell: GAP-0366) and the CSIR XII-FYP Project M2D (CSC-0134) for the grants received.

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