Elsevier

Materials Research Bulletin

Volume 103, July 2018, Pages 285-293
Materials Research Bulletin

Facile synthesis of various α-Fe2O3 micro/nanostructures: Highlighting on the enhanced catalysis activities by formation of bowl-like α-Fe2O3/Au composites

https://doi.org/10.1016/j.materresbull.2018.03.046Get rights and content

Highlights

  • Various α-Fe2O3 micro/nanostructures are fabricated by a hydrothermal method.

  • The morphologies are tunable by changing the content of NaHCO3 and reaction time.

  • Bowl-like α-Fe2O3 has the highest Fenton catalytic efficiency in dark condition.

  • Fast photo-Fenton reaction of α-Fe2O3/Au can be tuned by multiphase structures.

  • Bowl-like α-Fe2O3/Au is an excellent photo-Fenton catalyst for water treatment.

Abstract

Various α-Fe2O3 micro/nanostructures referring to spherical-like, bowl-like, cubic, and discoid shapes have been fabricated by a facile hydrothermal method using sodium bicarbonate as surfactant and alkaline reagents, displaying superior Fenton catalytic performances for degradation of Rhodamine 6G. Because of abundant exposed crystal surfaces such as (110) and (104) on the surfaces of bowl-like α-Fe2O3, the rapid degradation rate of 90.0% in 5 min is observed under the dark condition as the adding amount of H2O2 limiting to 300 μL. Significantly, the enhanced catalytic performances of bowl-like α-Fe2O3/Au catalysts under natural light condition have been investigated and carried out by in situ reduction of well-distributed Au nanoparticles with average sizes of 3–6 nm. Due to the appropriate phase structures and heterogenous interface properties, the highest photo-Fenton catalytic activity with the degradation rate of 90.0% in 2 min can be obtained for α-Fe2O3/Au0.01 composites.

Introduction

Recently, there have been continuing research interests for the development of different functional oxide micro/nanomaterials with high specific surface area, tunable sizes and shapes, and significant characteristic responses in the practical applications, such as heterogeneous catalysis, water treatment, biomedical imaging, drug delivery, energy storage, and sensing [1,2]. Although a lot of efforts have been concentrated on the morphology and structure controls of various oxide architectures, the apparent diversity and adjustability of micro/nanostructures achieved by facile synthetic routes have become the main limiting factors for understanding the fascinating relationship between the structure and property of products [3,4]. Actually, the investigation of new synthesis technologies employed for tuning microcosmic parameters played a dominant role for the configuration evolutions of advanced materials.

Iron oxides, as the important kind of semiconductor materials combining prominent functionality with low cost, low toxicity, the elevated specific surface area, and a high yield, have evoked considerable concentration as promising materials in the fields of catalysts, sensors, in-vivo magnetic imaging agents, magnetic fluids, and lithium batteries [[5], [6], [7]]. In particular, α-Fe2O3 micro/nanocrystals can be confirmed as good catalysts with remarkable surface effects in different reaction environments. The increase of surface active sites are mainly ascribed from the synergistic actions of small particle size, the volume fraction of surface states, the bonding and electronic states, and the surface atomic coordination [8]. α-Fe2O3 micro/nanocrystals with a variety of morphologies including spherical particles, nanorods, nanobelts, nanoplates, nanowires, nanotubes, and cubic shapes have been extensively investigated by numerous synthetic methods such as co-precipitation, electrodeposition, hydrothermal and solvothermal reaction, sonochemical reaction, and microwave-assisted synthesis in catalyst field [[9], [10], [11]]. For instance, Yin and co-workers prepared a series of α-Fe2O3 nanoparticles by a combination of co-precipitation and microwave drying technologies, exhibiting superior catalytic property for the ortho-methylation of phenol with methanol from 623 to 653 K [12]. Du and co-workers provided porous α-Fe2O3 nanocubes that were derived from Prussian blue nanocubes for improving the catalytic efficiency of the degradation of Rhdamine B (RhB) [13]. Yu and co-workers synthesized α-Fe2O3 hollow spheres with novel cage-like architectures and high surface area which were evaluated as good catalysts by photocatalytic decolorization of RhB aqueous solution at ambient temperature [14]. In general, the activity and selectivity of catalysts constituting of α-Fe2O3 particles can provide with the advantages of high efficiency and long-term operation. The catalysis properties of α-Fe2O3 products were depedent on the composition, structure, and size, as well as the surface exposed facet conditions. Until now, there have been an increasing attention on the influence of exposed facet on surface reactivity for catalysis process. For instance, Chen and co-workers proposed the mechanism study of redox mediator on α-Fe2O3 exposed by different facets and the enhanced photocatalytic water oxidation behavior [15]. The available fabrication of α-Fe2O3 micro/nanocrystals with tunable exposed facets and porous structure may open the door for further development of enhanced α-Fe2O3 calalytic systems.

Fenton catalytic process has been verified to be a high-efficiency approach for activating H2O2 to generate hydroxyl radical (OHradical dot) according to the radical (Weiss-Harber) mechanism, which has the second highest oxidant potential (about 2.7 eV) in nature and can completely oxidize organic compounds without light, heat, or electricity [16,17]. The design of α-Fe2O3-based heterogeneous Fenton catalysts would be of great importance for promoting the catalytic performance due to the rapid formation of strongly oxidizing reactive oxygen species with high reactivity and non-selectivity influenced by the structure optimization, dissolved oxygen, inorganic ions, and pH value. It is found that the introduction of precious metals onto the surface of α-Fe2O3 particles can alleviate the poor charge transport process and decrease the recombination of charge carriers through the tunable heterointerfaces in virtue of good catalytic activity, optical properties, and chemical properties [[18], [19], [20], [21], [22]]. Various inorganic heterostructures have been applied by introducing precious metals as the coating materials to inhibit charge recombination and make it possible for improving the catalytic activity. α-Fe2O3/Au catalysts can generally show attractive catalytic properties on different chemical reactions and detections, such as CO catalytic oxidation, NOX catalytic reduction, and propylene epoxidation [23]. Moreover, α-Fe2O3/Au micro/nanostructures with well-dispersed and shape-controlled Au nanoparticles can result in the enhancement of Fenton catalytic activity by suppressing the recombination and promoting the charge-transfer process of photogenerated electrons and holes at the photocatalyst/electrolyte interface under light irradiation [24,25]. On the other hand, the photo-induced electron ejection on the surface and the electron-transfer from Au to H2O2 can be commonly observed as Au atoms serving as the electron-donors or -acceptors under different environments, which is beneficial to the fast producing of OHradical dot for oxidation application [26]. Meanwhile, α-Fe2O3/Au heterostructures with a certain amount of Au deposition can probably induce the magnetic separation for recyclability, because of the phase transformation of iron oxides during the reduction of Au nanoparticles [27].

In this study, various α-Fe2O3 microstructures with tunable spherical-like, bowl-like, cubic, and discoid shapes have been synthesized by a simple and effective hydrothermal approach. The formation mechanisms and Fenton catalytic properties for degradation of Rhodamine 6G (R6G) were also investigated as well as the effect of different exposed crystal surfaces of α-Fe2O3 products on the catalytic activity. Furthermore, photo-Fenton catalytic systems of α-Fe2O3/Au composites have been obtained by adjusting the facile in situ reduction process. By optimizing the phase composition and surface configuration, α-Fe2O3/Au composites could exhibit excellent catalytic performance in photo-Fenton degradation of R6G.

Section snippets

Preparation of α-Fe2O3 micro/nanostructures

As for the bowl-like α-Fe2O3 products, 0.676 g of FeCl3·6H2O and 1.8 g of NaHCO3 were added into 25 mL of ethanol solvent to form the transparent solution under vigorous magnetic stirring about 30 min. The obtained solution was then transferred into a Teflon-lined stainless-steel autoclave, and subsequently sealed and heated at 200 °C for 16 h in an oven. After cooling to room temperature naturally, the products were collected and dried at ambient temperature after centrifugation by washing

Results and discussion

As shown in Fig. 1(a), the spherical-like nanoparticles with partial aggregation and large size distribution of 35–75 nm can be observed for Sample 1. Fig. 1(b) displays the uniform and well-dstributed spherical-like particles with rough surface in the range of 60–75 nm as the reaction time increasing to 8 h, along with the central section of particle surface starting to appear sinking phenomenon. It is obvious from Fig. 1(c and d) that bowl-like α-Fe2O3 particles with deeper and larger pore

Conclusions

A facile hydrothermal method has been developed to synthesize different α-Fe2O3 micro/nanostructures including spherical-like, bowl-like, cubic, and discoid shapes. Both the reaction time and the adding amount of NaHCO3 can play a vital role for tuning the phase structures and morphologies of α-Fe2O3 samples. The catalytic Fenton reaction for degradation of R6G results indicate that bowl-like α-Fe2O3 products have the highest catalytic efficiency of 90.0% in 5 min under the dark condition

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgments

This work was supported by the projects from the National Natural Science Foundation of China (51402123, 51402124, and 51602128), and the project from Shenzhen Gangchuang Building Material Co.,Ltd., and the National Training Program of Innovation and Entrepreneurship for Undergraduates (201610427017 and 201710427048), and Outstanding Young Scientists Foundation Grant of Shandong Province (BS2012CL006), and the scientific research fund of University of Jinan (XKY1301).

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