In the past decade, the fabrication of coordination polymers (CPs) and metal-organic frameworks (MOFs) with various nanostructures have attracted considerable interests due to their novel framework structures, topologies and interesting properties ^{[1, 2, 3, 4, 5]}. More recently, the use of CPs and MOFs with specific morphology as precursors for the synthesis of nanoscale metal oxides, metal oxide/amorphous carbon with interesting properties has received great attention ^{[6, 7, 8, 9, 10]}.

Indium oxide (In₂O₃), which is an n-type semiconductor with a direct band gap of 3.55-3.75 eV, has promising application in optoelectronic device, sensor and photocatalysis ^{[11, 12, 13, 14, 15, 16]}. In particular, In₂O₃ shows superior effect in photocatalytic degradation of dyes. Liu and co-workers reported In₂O₃ nanocubes with high photocatalytic activity for degenerating eosin B under UV irradiation ^[17]. Cao et al. synthesized hollow In₂O₃ nanocrystals with efficient photocatalytic degradation of RhB and MB under UV irradiation ^[18]. Zhang et al. reported In₂O₃ porous spheres possessed superior photocatalytic degradation of RhB under UV irradiation ^[19]. As the morphology of the nanomaterial is one of the key factors that affect the properties, the shape control of nanostructures has been investigated extensively. In recent years, various nano/micro-sized In₂O₃ with different morphologies have been synthesized ^{[17, 19, 20, 21, 22, 23]}. These nanostructures not only create intriguing variety of morphology features, but also provide good blocks for developing high photocatalytic performance.

In this work, we demonstrate a simple strategy for the synthesis of In(BTC)(phen)(H₂O) precursor without any additives. Just by modulating the volume ratio of DMF:H₂O:C₂H₅OH while keeping all the other conditions unchanged, a series of In(BTC)(phen)(H₂O) nanostructures with different sizes and shapes was obtained. After calcination in air, the as-prepared In(BTC)(phen)(H₂O) was subsequently converted to In₂O₃ nanostructures while their original morphologies were well maintained. Afterwards, photocatalytic performances of the as-obtained In₂O₃ were investigated and In₂O₃ nanocrystals exhibited excellent photocatalytic activity for the degradation of RhB molecules under UV light irradiation.

2.1. Sample preparation

Materials and chemicals: In(NO₃)₃·6H₂O, 1, 3, 5-benzenetricarboxylic acid (H3BTC), 1, 10-phenanthroline (phen), N, N-dimethylformamide (DMF) and ethanol (C₂H₅OH) were obtained from Nanjing Chemical Reagent Co., Ltd. All chemicals are analytically pure and were used as received without further purification.

In a typical experiment, In(BTC)(phen)(H₂O) was prepared by procedures reported previously ^[24]. In(BTC)(phen)(H₂O) nanostructureswere obtained by solvothermal reaction of In(NO₃)₃·6H₂O (18.4mg, 0.045mmol), H₃BTC (9.5 mg, 0.045mmol), phen (8.9 mg, 0.045mmol) and 30 mLsolvent ofDMF, H₂O and C₂H₅OHin a 43mL Teflon-lined autoclave, which was sealed and maintained at 130 ℃ for 4 h. The reaction vessel was then removed from the oven and allowed to cool to roomtemperature. The particleswere isolated by centrifugation and washed several times withDMF and C₂H₅OHand dried in a vacuum at 60 ℃ for 5 h. Anal. Calcd. for C₂₁H₁₃N₂O₇In: C 48.45, H 2.50, N 5.38%. Found: C 48.30, H 2.18, N 5.64%.

2.1.2. Synthesis of In₂O₃ nanocrystals

The as-obtained precursor was calcinated in air at 400 ℃ for 1.5 h with a heating rate of 2 ℃ min^-1 to produce In₂O₃ nanostructures.

Powder X-ray diffraction (PXRD) data were obtained on a Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å ). Scanning electron microscopy (SEM) images were measured on a field emission scanning electron microanalyzer (Hitachi S-4800), employing an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) images were obtained on a JEM-2100 high resolution transmission microscope (HRTEM), employing an accelerating voltage of 200 kV. FT-IR spectra were recorded in the range of 400-4000 cm^-1 on a Bruker Vector22 FT-IR spectrophotometer using KBr pellets. Elemental analysis for C, H, and N was performed on a Perkin- Elmer 240C Elemental Analyzer. Thermogravimetric analyses (TGA) were performed on a Mettler-Toledo (TGA/DSC1) thermal analyzer under the nitrogen atmosphere with a heating rate of 10 ℃ min^-1. The UV-vis spectra were recorded on a Shimadzu UV-3600 spectrophotometer. Sorption experiments at 77 K were carried out on a Belsorp-max volumetric gas sorption instrument. Surface areas were determined by the BET equation. Prior to measurement of sorption, the samples were activated under vacuum at 100 ℃ for about 10 h.

The photocatalytic activity of the as-obtained crystalline In₂O₃ nanomaterials was evaluated by photodegradation of rhodamine B (RhB) dye aqueous solution under UV irradiation. A 300WHg arc lamp was used as a light source to provide the UV light at the wavelength of 365 nm. In₂O₃ (40 mg) was dispersed into rhodamine B aqueous solutions (40 mL) with a concentration of 1 × 10^-5 mol L^-1 and irradiated by UV light for different time at room temperature.

Fig. 1 shows the PXRD patterns of the In(BTC)(phen)(H₂O) nanostructures. The data imply the unique crystalline phase, and are in good agreement with the simulated one from the single crystal data ^[23]. The FT-IR spectral data further support the chemical composition of the In(BTC)(phen)(H₂O) nanostructures (Fig. 2). The characteristic bands around 1608 and 1342 cm^-1 are the stretching vibrations of ν_as(-COO^-) and ν_s(-COO^-) of the carboxylate in BTC^3-. The TGA curve of the In(BTC)(phen)(H₂O) nanostructures displays two main steps of weight loss (Fig. 3). The first weight loss from 270 ℃ to 310 ℃ is attributed to the loss of coordinated water molecules and the second one is due to the decomposition of the In(BTC)(phen) framework from 340 ℃ to 470 ℃.

Fig. 1

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Fig. 1.XRD patterns of simulated In(BTC)(phen)(H2O) and the as-obtained precursors with different volume ratio of DMF:H2O:C2H5OH: (a) simulated, (b) 1:1:1, (c) 2:1:1 and (d) 4:1:1.

Fig. 2

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Fig. 2.FT-IR spectra of the as-obtained precursors with different volume ratio of DMF:H2O:C2H5OH: (a) 1:1:1, (b) 2:1:1 and (c) 4:1:1.

Fig. 3

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Fig. 3.TGA curves of In(BTC)(phen)(H2O) nanocrystals.

Effectof solventonthe formationof In(BTC)(phen)(H₂O) particles was investigated, and it was found that the reaction solvent played a crucial role in the formation of particles with different sizes and shapes. Fig. 4 shows field emission SEM images of In(BTC)(phen)(H₂O) nanostructures under different volume ratio of DMF:H₂O:C₂H₅OH.When the volume ratio of DMF:H₂O:C₂H₅OH is 1:1:1, the uniform hierarchical straw-sheaf-like architectures made from an abundance of nanorods with diameters of 50 nm and lengths of 100 nm are obtained and the average particle sizes are around 5 mm(Fig. 4a and b). The crystal aggregate was dispersed to the uniform nanorods with average lengths of about 1 mm and 200 nmin major and minor axes, respectively (Fig. 4c and d), when theDMF:H₂O:C₂H₅OHratio increases to 2:1:1.Withfurther increase of the DMF:H₂O:C₂H₅OH ratio to 4:1:1, the elongated hexagonal particles are obtained with average lengths of about 2.5 mm and 600 nminmajor andminor axes, respectively (Fig. 4e and f). In short, with increasing the DMF content in the mixed solvent, hierarchical straw-sheaf-like architectures dispersed to small monodisperse nanorods first, and then those nanorods gradually increased in size and appeared in elongated hexagonal shapes. Apparently, the size of crystal can be easily tuned by the volume ratio ofDMF:H₂O:C₂H₅OH. It indicates that DMF plays an important role during the crystal growth process and affects the crystal nucleation and growth rate, which can be explained by changing the solubility ofmetal salts and ligands in the mixed solvent. Generally speaking, the pattern formed by crystallization is generally affected by the balance of the driving force for the crystallization and the diffusion rate of ions or molecules ^{[25, 26]}. When increasing the content of DMF, the solubility of metal salts and ligands is raised, and thus the diffusion rates are accelerated, which decreases the driving force for the crystallisation relative to that for diffusion; thus, the ligands could not link themetal ionsimmediately and thenucleationratebecomes slow, larger size particles were obtained.

Fig. 4

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Fig. 4.SEM images of the as-obtained precursors with different shapes: (a and b) straw-sheaf-like architectures, (c and d) nanorods and (e and f) elongated hexagons.

On the basis of the results of TGA, it was found that the proper temperature to obtain In₂O₃ fromIn(BTC)(phen)(H₂O) is 400 ℃. As a result, the thermal decomposition of the corresponding In(BTC)(phen)(H₂O) nanocrystals was performed at 400 ℃. After being annealed at 400 ℃ for 1.5 h in air, the as-synthesized In(BTC)(phen)(H₂O) nanocrystals were completely converted to phase-pure In₂O₃. Fig. 5 shows the morphology of the as-obtained products, it can be clearly seen that the original shape can be maintained after calcination. Fig. 5g shows a representative TEM image of the In₂O₃ nanorods. From high-magnification TEMimages (Fig. 5h), the inter-planar distance of 0.503 nmis in good agreement with the (2 0 0) plane of cubic In₂O₃. The crystallographicphase of all threesamples is again examined byPXRD(Fig. 6). All the peaks are in good accordance with the standard spectrum of In₂O₃ (JCPDS 06- 0416), which means that the precursor has been completely transformed into the In₂O₃ nanostructures.

Fig. 5

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Fig. 5.SEM images of the as-obtained In2O3 with different shapes: (a and b) straw-sheaf-like architectures, (c and d) nanorods, (e and f) elongated hexagons, (g) TEM and (h) HRTEM of as-prepared In2O3 nanorods.

Fig. 6

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Fig. 6.Standard spectrum of In2O3 (a), XRD patterns of as-obtained In2O3 with different shapes: (b) straw-sheaf-like architectures, (c) nanorods and (d) elongated hexagons.

The photocatalytic activities of the as-obtained In₂O₃ products were investigated by degrading the RhB solution under UV light irradiation. Fig. 7 shows the comparison of photocatalytic activities among the In₂O₃ products with different morphologies. In the absence of catalysts, the photolysis of RhB under UV light was almost negligible, less than 7% after irradiation for 8 h. In addition, the In₂O₃ products with hierarchical straw-sheaf-like architectures, nanorods and elongated hexagonal shapes show the degradation efficiencies of 86%, 94%, and 89%, respectively. It is known that the photocatalytic activity of metal oxide particles is closely related to their surface area and pore volume ^{[16, 18, 27]}. Therefore, the more effective photocatalytic activity of In₂O₃ nanorods may be attributed to the larger BET surface area and pore volume, which provides more photocatalytic sites. To investigate the BET surface area of the In₂O₃ nanostructures with different morphologies, nitrogen sorption measurements were performed. Nitrogen adsorption-desorption isotherms of the asobtained products are shown in Fig. 8. The BET surface areas and pore volumes of the products are 28.3 m² g^-1 and 109.2 mm³ g^-1, 33.4 m² g^-1 and 124.3 mm³ g^-1, and 29.6 m² g^-1 and 120 mm³ g^-1 for hierarchical straw-sheaf-like architectures, nanorods and elongated hexagons, respectively. Therefore, the highest photocatalytic capacity of nanorods may be attributed to the largest BET surface area and pore volume, which is beneficial for the degradation of RhB.

Fig. 7

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Fig. 7.Normalized degradation curves of RhB solution with the (a) blank test, asobtained In2O3 with different shapes: (b) straw-sheaf-like architectures, (c) nanorods and (d) elongated hexagons.

Fig. 8

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Fig. 8.N2 adsorption-desorption isotherms of the as-obtained In2O3 with different shapes at 77 K: (a) straw-sheaf-like architectures, (b) nanorods and (c) elongated hexagons.

In summary, this work explored an effective strategy for tuning the size and morphology of In(BTC)(phen)(H₂O) crystals by changing the volume ratio of DMF:H₂O:C₂H₅OH. In the presence of DMF, crystal size and shape can be well tailored upon variation of the volume ratio of DMF:H₂O:C₂H₅OH, resulted in hierarchical straw-sheaf architectures, nanorods and elongated hexagonal crystals with different sizes. After converting the samples into pure cubic In₂O₃ by thermal annealing, the obtained products show no substantial morphological alternation. The In₂O₃ samples have been demonstrated to have good photocatalytic properties in the degradation of RhB. Furthermore, In₂O₃ nanorods have the highest photocatalytic activity, which is attributed to the large BET surface area and pore volume.

This work was financially supported by the National Natural Science Foundation of China (No. 21331002). This work was also supported by a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.