Synthesis of small Fe₂O₃ nanocubes and their enhanced water vapour adsorption–desorption properties

Abstract

It is of great importance to develop humidity-buffering materials and thus to improve indoor environments. Uniform Fe₂O₃ nanocubes with an average pore size distribution of 8.48 nm exhibit superior water adsorption–desorption ability, arising from the appropriate pore size distribution around the condensation critical radius. This finding can provide a novel strategy to design and prepare new humidity-buffering nanomaterials.

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Water adsorption technologies have attracted a tremendous amount of interest due to their extensive applications in construction fields, pharmaceutical processing, food industry, microelectronics, and so on. Examples include desiccant dehumidifiers, the adsorption heat transformation, fresh water production from the air, adsorptive air-conditioning systems, etc.¹ Humidity-buffering materials are key components for water adsorption–desorption technologies. Various new compounds and materials for humidity-buffering materials have been reported in the past few decades, including zeolites, hydrophilic polymers, silica gel, activated carbon, aluminosilicate materials, biomass-based materials, metal–organic frameworks (MOFs), diatomite materials, and so forth.² Among them, porous humidity-buffering materials have attracted plenty of attentions because of their high surface areas, ordered pore distributions, good surface chemistry and high pore volumes.³

As is well known, small-sized inorganic nanocrystals with large specific surface areas have got extensive research interests owing to the possibility of tuning their optical, chemical and electronic property on a large scale, which can cause the tunable multi-functionalities.⁴ α-Fe₂O₃, as the most stable iron oxide under ambient atmosphere, has sparked increasing interest over the past few decades because of its low cost, high corrosion resistance, abundant and eco-friendly properties.⁵ To our knowledge, the potential application of α-Fe₂O₃ porous nanomaterials as a stand-alone material for adsorbents has not been studied yet. Herein, preparation and water vapour adsorption–desorption property of the well-dispersed α-Fe₂O₃ nanocubes in a narrow size distribution and large specific surface area are our main objectives. Nanoparticles exhibit properties different from those of their bulk counterparts, which can be exploited to obtain new functionalities.⁶

In this work, we presented a facile and simple one-pot hydrothermal approach to prepare α-Fe₂O₃ nanocubes with an average pore size distribution of 8.48 nm. The size of the nanocube is around 40–50 nm. The uniform ordered cube-like Fe₂O₃ nanomaterial showed an excellent humidity-buffering ability, due to adsorption and desorption of water vapour by utilizing the intra- and inter-granular pores and large specific surface areas. Specifically, the Fe₂O₃ nanocubes has a much higher water adsorption capacity compared with bulk Fe₂O₃ counterparts and porous Fe₂O₃ nanofibers. Further comparison with the conventional humidity-buffering materials (raw diatomite and calcined diatomite) was conducted. Results show that the significantly enhanced water adsorption capacity of Fe₂O₃ nanocubes mainly arises from the appropriate pore size distribution nearby condensation critical radius.

The structure and composition of the as-prepared powder samples was characterized by XRD analysis, as shown in Fig. 1. All the diffraction peaks can be identified to the Fe₂O₃ phase (JCPDS card no. 33-0664). No other characteristic peaks from the other crystalline impurities were detected, thus indicating formation of the pure Fe₂O₃ products. The morphology and size of the as-prepared Fe₂O₃ products were visualized by SEM. The low-magnification SEM image (Fig. 2A) shows that the product is composed of uniform ordered nanocubes. The high-magnification SEM image (Fig. 2B) clearly reveal that, the cube-like shape have a narrow size distribution of 40–50 nm. Additionally, the chemical composition of these nanocubes is further investigated by energy-dispersive spectroscopy (EDS, inset in Fig. 2A). The strong peaks of Fe and O are observed in the EDS result, and the atomic percentage ratio is about 2 : 3, which is close to the stoichiometry of Fe₂O₃. For comparison, bulk counterpart Fe₂O₃ powder and Fe₂O₃ porous nanofibers were also measured.⁷ The structure and morphology results were shown in Fig. S1.† Pure Fe₂O₃ crystalline phases (JCPDS card no. 33-0664) were identified. The morphology of commercial Fe₂O₃ powder was nanocables with particles, of which average particle size was about 100 nm. The morphology of uniform Fe₂O₃ porous nanofibers can be observed obviously.

Fig. 1 XRD pattern of the as-synthesized Fe₂O₃ nanocubes.

Fig. 2 (A) Low-magnification SEM image of the Fe₂O₃ nanocubes, and the EDS pattern (inset); (B) enlarged SEM image of the Fe₂O₃ nanocubes.

Further insight into the microstructure and morphology of cube like Fe₂O₃ nanoarchitecture was obtained through TEM and HR-TEM. Fig. 3A shows the low-magnification TEM images of the Fe₂O₃ nanostructures, uniform ordered small-sized nanocubes can be observed clearly, in accordance with the SEM results (such as Fig. 2B). The high-magnification TEM images in inset of Fig. 3A and B reveal the presence of mesopores in the nanocubes. The HR-TEM image (Fig. 3C), SAED (inset) and FFT pattern (Fig. 3D) exhibit the high-crystalline characterization with a lattice spacing of 0.37 nm, which can be indexed readily to (101) plane of Fe₂O₃ nanocubes.

Fig. 3 (A) Low-magnification TEM image of the Fe₂O₃ nanocubes, inset shows a magnified nanocube to emphasize the internal nanoporosity; (B) enlarged TEM image of the Fe₂O₃ nanocubes; (C) HR-TEM image and SAED pattern (inset) of the nanocubes. (D) FFT pattern of the edge area of nanocubes.

The specific surface area and porosity of the Fe₂O₃ samples were investigated. Fig. 4 shows the N₂ adsorption/desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution curve of the as-obtained Fe₂O₃ nanocubes, nanofibers and bulk Fe₂O₃ counterpart powder. Results show that the isotherm of Fe₂O₃ nanocubes can be categorized as type IV, with the distinct hysteresis loop observed in the range of 0.5–1.0 p/p₀ on the basis of IUPAC classification. Evidently, the pore size distribution is relatively narrow, with an average size of 8.48 nm. This result coincides with that of the TEM (Fig. 3A and B). The BET surface area and pore volume of Fe₂O₃ nanocubes are calculated to be 50.91 m² g⁻¹ and 0.17 cm³ g⁻¹ respectively, which are much higher than that of bulk Fe₂O₃ counterpart powder (8.79 m² g⁻¹, 0.02 cm³ g⁻¹) and porous Fe₂O₃ nanofibers (13.4 m² g⁻¹, 0.15 cm³ g⁻¹).

Fig. 4 N₂ adsorption–desorption isotherm for (A) the Fe₂O₃ nanocubes, bulk Fe₂O₃ and (B) Fe₂O₃ nanofibers. The inset shows BJH pore size distributions of the samples.

Such a high specific surface area is believed to benefit from the synthesis process of nanocubes.⁸ Control experiments were carried out to determine the synthesis parameters that may affect the formation of Fe₂O₃ nanocubes. We found that the addition of urea and DEG has strong effects on the formation of nanocubes. Fig. S2† shows SEM images of the samples obtained when different amounts of urea are introduced into the reaction system. Without using urea, microrods with lengths of about 10 μm are the dominant products in the batch solution. Increasing the amount of urea, more and more nanocubes appear in the product together with a decreased amount of rod-like microstructures. The major role of urea can be assigned to provide OH⁻ ion supply in aqueous solution through CO(NH₂)₂ hydrolysis.⁹ Fig. S3† gives SEM images of the products obtained when EG and PEG 400 replace DEG while keeping the other conditions unchanged. It is obvious that nanocubes can be fabricated only under the presence of DEG. As reported in literature, DEG with its high boiling point and viscosity property has been widely used in the polyol method to fabricate a series of monodisperse nanostructured materials.¹⁰

Comparative experiments were carried out to examine the effect of Fe₂O₃ nanomaterials on the water vapour adsorption–desorption. As shown in Fig. 5, the adsorption capacity of water vapour at both low and high humidity ranges follows the order of bulk Fe₂O₃ < Fe₂O₃ nanofibers < Fe₂O₃ nanocubes. This result is consistent with the trend of pore size, pore volume and surface area of Fe₂O₃ nanomaterials. In addition, conventional humidity-buffering materials (raw diatomite and calcined diatomite) were prepared and used as control samples. Their N₂ sorption isotherm, BJH pore size distribution and water sorption kinetic are shown in Fig. S4 and S5.† The results of BET surface areas, BJH average pore sizes, pore volumes values and the maximum adsorption capacity of water vapor of the Fe₂O₃ nanocubes, Fe₂O₃ porous nanofibers, bulk Fe₂O₃ counterparts, raw diatomite and calcined diatomite samples were summarized in Table 1. The adsorption capacity of the Fe₂O₃ nanocubes was higher than that of the raw and calcined diatomites adsorbents. More interesting, similar BET surface areas were observed between the Fe₂O₃ nanocubes and calcined diatomite sample, but the water adsorption capacity of Fe₂O₃ nanocubes was found to be much higher than that of calcined diatomites adsorbent. In general, BET surface area is one of the most critical influences on water vapour adsorption performance.¹¹ Our experimental results demonstrate that there are some other influences on the water vapor adsorption efficiency, besides BET surface area in this system.¹² Considering the samples with different pore structure, pore size may be another big factors influencing their water vapour adsorption performance. For Fe₂O₃ nanocubes, the largest increase in uptake takes place in the range of medium RHs when the filling of micropores starts, and follows at the higher RHs by capillary condensation in the mesopores. At the relatively low water vapour pressure, the water is preferentially adsorbed on the highly hydrophilic sites, formed monolayer adsorption.¹³ At the relatively higher pressure, multilayer adsorption occurs, followed by capillary condensation in the porous channels.¹⁴

Fig. 5 Water adsorption–desorption kinetics and isotherms at room temperature of the Fe₂O₃ nanocubes (A and B), bulk Fe₂O₃ (C and D) and Fe₂O₃ nanofibers (E and F) samples equilibrated for a fixed time.

Table 1 BET surface areas, BJH average pore sizes, pore volumes and maximum water adsorption capacity values of Fe₂O₃ nanocubes, Fe₂O₃ nanofibers, bulk Fe₂O₃ counterparts, raw diatomites and calcined diatomites

Sample	Surface area (m^2 g^−1)	Pore size (nm)	Pore size (cm^3 g^−1)	Max water sorption capacity (%)
Fe2O3 nanocubes	50.91	8.48	0.17	14
Fe2O3 nanofibers	13.4	35.2	0.15	0.8
Bulk Fe2O3	8.79	6.78	0.02	0.4
Raw diatomite	71.57	9.32	0.10	10
Calcined diatomite	52.45	6.75	0.09	64

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For the capillary condensation phenomena, Kelvin equation can be used. At the higher RH, the pore diameter, where capillary condensation takes place, can be calculated via Kelvin equation:

where p is the vapor partial pressure; p₀ is the saturated vapour pressure; σ is the surface tension, V_L is the molar volume of the liquid, R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), γ_m is the Kelvin radius of the mesopores, and T is Kelvin's temperature. Corresponds to the capillary condensation, the saturation takes place in all the pores when radii is up to γ_m, under a constant pressure and temperature. Considering multi-layer adsorption, the respective average pore diameter for 75% humidity, using the corrected Kelvin equation, is about 8.5 nm (At this condition, the pore diameter for 75% humidity is about 8.5 nm). Therefore, capillary condensation can be observed in the as-prepared Fe₂O₃ nanocubes with an average pore diameters of 8.48 nm. It means that the excellent humidity sensitivity obtained for the uniform Fe₂O₃ nanocubes can be explained by the favorable pore size distribution nearby condensation critical radius. Additionally, from the point of the irreversible chemisorption and reversible physisorption, this water vapour adsorption–desorption of Fe₂O₃ nanocubes is physisorption, due to the high stability of the water vapor adsorption–desorption capacity (Fig. 6).¹⁵ Hence, the difference of water vapour adsorption of the samples was mainly dependent on the pore structures rather than surface chemical properties since many suspended oxygen bonds exist in this case.¹⁶ Although further investigation is necessary to elucidate the water absorption–desorption mechanism of Fe₂O₃ nanocubes, we hope the present study will stimulate new investigations of the relationship between nanostructure materials and water vapour adsorption–desorption.

Fig. 6 Cycling runs for the water vapour adsorption–desorption over Fe₂O₃ nanocubes at room temperature.

Conclusions

In summary, we have successfully demonstrated Fe₂O₃ nanocubes with an average pore size distribution of 8.48 nm exhibits excellent humidity-buffering ability for the first time. The as-synthesized Fe₂O₃ nanocube samples show much higher water vapour adsorption/desorption performance than bulk Fe₂O₃ counterparts, porous Fe₂O₃ nanofibers and conventional humidity-buffering materials (raw and calcined diatomites). The superior water adsorption/desorption performance of Fe₂O₃ nanocubes can be attributed to the favorable pore size distribution around condensation critical radius. The present work might open new doors for the exploration of uniform ordered nanomaterials with excellent water vapour adsorption/desorption ability.

Acknowledgements

The work was grateful to the financial aid from NSFC (51402047 and 5152500382), the Fundamental Research Funds for the Central Universities (N130410002 and L1502017), Doctoral Program of Higher Education of China (20130042120011), Open Project of State Key Laboratory of Rare Earth Resource Utilizations (RERU2014002).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental procedures; XRD and SEM image of the bulk Fe₂O₃ powder and porous Fe₂O₃ nanofibers; SEM images of the Fe₂O₃ samples with different amounts of urea; SEM images of products prepared under similar conditions but with DEG replaced by EG, and PEG 400; N₂ adsorption–desorption isotherm for the conventional samples (raw diatomite and calcined diatomite). The inset shows BJH pore size distributions of the samples; water sorption kinetics of the conventional samples (raw diatomite and calcined diatomite) equilibrated for a fixed time at room temperature. See DOI: 10.1039/c5ra12456e

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