Facile fabrication of flower like self-assembled mesoporous hierarchical microarchitectures of In(OH

A template and capping-reagent free facile fabrication method for mesoporous hierarchical microarchitectures of flower-like In(OH) 3 particles under benign hydrothermal conditions is reported. Calcination of In(OH) 3 to In 2 O 3 with the retention of morphology is also described. Both In(OH) 3 and In 2 O 3 microstructures were analyzed with SEM, EDX, TEM and powder X-ray diffraction. The crystal sizes for In(OH) 3 and In 2 O 3 were calculated using the Scherrer ** * * Corresponding author.


Introduction
Micro structured materials show a clear correlation between their morphology and function, which in turn decides their applications.Micro/superstructures with good interconnectivity and conductivity are useful in several fields [1].As size and shape of materials are controlling factors, thereby influencing to the physical and chemical properties, it is natural that the synthesis of materials with fascinating morphologies is of researchers' interest.In particular, porous hierarchical architectures with interesting applications are materials of wide importance.Synthesis methods based on a hydrothermal process are especially beneficial due to its simplicity and modifiability, and also it is environmentally benign.In surfactant-free, simple hydrothermal process crystals as large as a few micrometers can be obtained simply by extending the reaction period under controlled conditions.

Hydrothermal synthesis and calcination
Milli Q-Plus water (resistance = 18.2 M•Ω) was used for all experimental work.In(OH) 3 microstructures with a flower-like morphology were prepared using indium nitrate (3 g, 10 mmol) and biuret (6.2 g, 60 mmol).Both were separately dissolved in about 100 mL of water and heated to the boiling point.The hot solutions were mixed under stirring (200 rpm), and after 30 min the mixture was transferred into a 250 mL Teflon cup.The sealed Teflon vessel was put into a stainless steel flask and kept in an autoclave at 200 °C for 15 h.After hydrothermal reaction, the flask was allowed to cool to room temperature and the precipitate was filtered, washed with water and alcohol to eliminate the possible impurities.It was then dried in an air oven at 120 °C for 2 h.In the next step In 2 O 3 micro flowers were prepared by calcining the precursor at 450 °C for 3h in a hot air oven.The oven was allowed to cool down to room temperature and the sample was collected.In an alternative process, first the reaction temperature was decreased respectively to 150 °C within 10 h time.The total quantity of water was about 150 ml for preparing In(OH)3 microcubes.The subsequent calcination was made at 450 °C for 3h to yield porous In 2 O 3 microcubes.This paper focuses only on the characterization of mesoporous hierarchical microarchitectures of In(OH) 3 and In 2 O 3 , as micro cube morphology has been well reported [9,16,18,19].The experimental part reports only the optimized experimental conditions in which uniform morphologies are observed, and the conditions which yielded inhomogeneous morphologies were deliberately avoided.

Characterization
The morphology was examined using a Hitachi S-4100 scanning electron microscope (SEM).Prior to SEM measurements, the samples were mounted on a carbon platform that was then coated with platinum using a magnetron sputter for 10 minutes.The In/O ratio within the sample was analyzed using an energy-dispersive X-ray spectrophotometer installed in the SEM.For the transmission electron microscope (TEM) study, the sample was dispersed onto a Cu grid with holey carbon supporting film and studied at room temperature in a FEI Tecnai 12 microscope operated at 120 kV.The powder X-ray diffraction (XRD) analysis was carried out using a Bruker D8 Advance diffractometer (Cu K α1 source, λ= 1.5406 Å) with step size of 0.01° 2θ and step time of 0.5 sec.The specimen was prepared on a silicon zero-background holder using petrolatum jelly as an adhesive.
The Scherrer equation was used for assaying the crystal size of the substances.Obtained values were corrected by the instrumental broadening which was determined by highly crystalline corundum sample measured with the same instrument settings.
wherein D stands for average size of the crystals, K as shape-dependent Scherrer's constant (0.9), λ as radiation wavelength (1.5406 Å), and β as FWHM of the peak profile subtracted by instrumental broadening; given in radians.Qualitative and semi-quantitative phase analysis (reference intensity ratio method; RIR) and Scherrer equation based crystal size estimations were all made by the HighScore Plus program v 4.5 [20].
Nitrogen sorption isotherms were measured at 77 K using a Quantachrome Nova-1000 instrument.The Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) model were used for specific surface area calculation and porosity evaluation, respectively.The sample was degassed at 175 °C before analysis.

SEM and TEM studies
The hydrothermal reaction products present flower-like morphology in the diameter range of 2 to 3.5 µm, as evidenced by SEM images (Fig. 1).These microstructures are 3D hierarchical structures assembled from nano-sized flakes with the thickness of about 30 to 50 nm (Fig. 1d).The results from the EDX spectrum (Fig. SM1) suggest formation of In(OH) 3 as the determined In/O ratio is close to 1:3; which is further confirmed unambiguously by the powder X-ray diffraction (see following sections).TEM images (Fig. 2) of the hydrothermal products indicate that the micro flowers are composed of thin flakes which are devoid of specific size.These flakes at the periphery of the micro flowers are electron beam sensitive.When the sample was irradiated with high energy electron beam (TEM), pore structures are formed due to decomposition, as shown in Fig. 2e-f.It should also be noted that the similar flake-like structures in In 2 O 3 micro flowers are stable under TEM experimental conditions (discussed subsequently), and hence the formation of pores under electron beam for In(OH) 3 microstructures may be due to a dehydration process induced by the beam.It is interesting to note that the similar decomposition (and thereby the formation of porous structures, which are called mesocrystals

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6 [21]) has been reported recently [16].These mesocrystals or otherwise known as colloidal crystals and are found to have interesting applications due to their larger surface area [16].The EDX spectrum of calcination product (Fig. SM2) indicates that In 2 O 3 has been formed as the In:O ratio is close to 2:3.From SEM images (Fig. 3) it can be observed that porous microstructures were formed as, by calcination with the retention of flower like morphology.It was also observed that the micro flowers are composed of nano flakes which are thin and curly

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(Fig. 3f) instead of irregularly shaped as in the case of the precursor (In(OH) 3 ).Highmagnification TEM images (Fig. 4) (Fig. 4d) evidence the porous nature of calcinated product.

Powder XRD analysis
The powder X-ray diffraction pattern of the hydrothermal product indicates that its main component is cubic In(OH) 3 , as all the significant diffraction peaks correspond to the ICDD [22] reference pattern 01-085-1338 (Fig. 6a) [23].In addition, few weak peaks e.g. at 25.93° and 33.97° 2θ indicate that a small amount of orthorhombic InO(OH) [ICDD: 04-010-2484] exist in the sample [24].Simple semi-quantitative analysis with the reference intensity ratio method (RIR) indicates about 3 % weight fraction for the minor component.Typically InO(OH) is regarded as a metastable form.However, it has been reported that excess of OH -ions is key factor for favoring formation of In(OH) 3 over InO(OH) [16], which in our case may have been the compensating factor for the pressurized conditions prevailing in the hydrothermal process.microflowers exhibited a hysteresis loop in a high relative pressure range of 0.57-0.97,which is characteristic of type IV isotherm, evidenced the presence of mesopores.The BET surface area is found to be 46.84m 2 g -1 .The total pore volume is 0.07921 cm 3 g -1 and the average pore diameter is 67.6 Å.The observed results exemplified the mesoporous nature of In 2 O 3 micro flowers.

Role of biuret and product formation
The triple role of urea (surface anchored organic molecule; chelating agent) as an alkaline medium has been recently been reported [9], and in this study biuret is used instead as it is expected to have a similar role.The formation of In(OH) 3 is expected to proceed according to equations 1-3, and calcination of In(OH) 3 to In 2 O 3 by eq. 4.
In both the cases, after reactions under hydrothermal conditions, the resulting In(OH) 3 is supersaturated in the solution.Followed by nucleation (Fig. 7), the self-assembly of particles proceeds in such way that surface energy will be minimum and hence the thermodynamically driven Ostwald ripening could be the process which results in stable larger crystals.The energetically unfavorable small particles are dilute in solution thereby easing the formation of larger (micro) structures with least surface energy.It was already reported that the similar process was the key factor behind the formation of cubic In(OH) 3 microstructures [9] and it is the key process which determines the morphology of the final products in template free hydrothermal reactions [26].
and In 2 O 3 structures, herein we report template and capping-reagent free hydrothermal synthesis of flower like In(OH) 3 microstructures and its M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 3 conversion to In 2 O 3 .To the best of our knowledge this is the first report on the synthesis of mesoporous hierarchical microarchitectures with a flower-like morphology for both In(OH) 3 and In 2 O 3 .

Fig. 2 .
Fig. 2. TEM images of (a-b) In(OH) 3 micro flowers; (c-d) Flakes at the periphery of the micro These pores are due to dehydration from In(OH) 3 .The low-magnification TEM images (Fig. 4ac) exemplify the flower like morphology and the thick nucleus of the microstructures.The SEM images of In(OH) 3 and In 2 O 3 micro cubes are shown in Fig. 5a and b respectively.As discussed in the experimental part the discussions were limited to micro flower architectures.

Fig. 3 .
Fig. 3. SEM images of (a-b) In 2 O 3 micro flowers; (c-f) Thin and curly nano flakes present in

Fig. 4 .
Fig. 4. TEM images of (a-b) In 2 O 3 micro flowers; (c) Flakes at the periphery of micro flowers; (d) Porous petals of In 2 O 3 micro flowers (pores formed by calcination).

Fig. 6 .
Fig. 6.Experimental powder X-ray diffraction patterns of a) In(OH) 3 and b) In 2 O 3 micro flowers.

Fig. 7
Fig. 7 shows the typical sorption isotherms of In 2 O 3 microflowers.The prepared In 2 O 3