Tailored synthesis of well-faceted single crystals of Fe₃O₄ and their application in p-nitrophenol reduction

Abstract

Well-defined Fe₃O₄ nanocrystals (NCs), which exhibit excellent catalytic properties, have been synthesized in high yield via a facile hydrothermal method. Both the type of hydramine and concentration of NaOH play important roles in the formation of the Fe₃O₄ NCs with various morphologies. Detailed investigations indicated that the cubic and polyhedral Fe₃O₄ NCs can be prepared by adjusting the concentration of the triethanolamine (TEOA) in solution. The addition of increasing amounts of NaOH was found to facilitate the morphology transition from Fe₃O₄ cube shape to octahedron shape. The catalytic performances of these prepared samples have been investigated by reducing p-nitrophenol into p-aminophenol by an excess of N₂H₄ without the assistance of noble metals. The kinetics of the reduction reaction at different temperatures was also investigated to determine the activation parameters. Importantly, the obtained (100) faceted Fe₃O₄ nanocubes exhibits significantly higher catalytic activities than (111) faceted Fe₃O₄ octahedrons and bulk Fe₃O₄ nanoparticles, and these catalysts are stable and can be easily recycled.

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Introduction

The catalytic hydrogenation of p-nitrophenol to its corresponding p-aminophenol is one of the crucially important organic reactions in the synthesis of chemicals and intermediates. In the past few years, various catalysts have been developed including noble metals (e.g., Pd, Pt, Au, Ag),^1–4 metal alloys (e.g., Au–Pt, Ru–Pt, Pt–Pd–Ni),^5–7 noble metal and metal oxides coupling (e.g., Au/Fe₃O₄, Au/CeO₂, Ag–Pd/Fe₃O₄),^8–10 and metal oxides (e.g., CuO/γ-Al₂O₃, Fe₃O₄, NiCo₂O₄),^11–13 etc. In contrast with expensive noble metals and noble metal-containing composites, metal oxides with higher reactivity along with other advantages such as thermal stability, chemical stability, low cost, easy recovery and reusability may become more and more favored.

Owing to wide-ranging industrial applications and fundamental importance, tailored synthesis of well-faceted single crystals of metal oxides has long been an attractive field in nanoscience and technology.¹⁴ The various facets or directions in a crystal may exhibit different physical and chemical properties. Generally, the properties of nanocrystals can be finely tuned, spanning a range of applications, by their shape to determine surface atomic arrangement and coordination.¹⁵ For example, the surface properties of materials highly depend on the shape of the nanocrystals and have great influence on the catalytic activity of nanocrystals in chemical reactions.¹⁶ Thus, unprecedented research efforts have been focused on the controllable preparation of micro-/nanocrystals with various geometries and exposed surfaces.

Among metal oxides, iron oxide (Fe₃O₄) are emerging as promising cost effective materials in information recording, photoelectrical devices, magnetic fluids drug deliver, in vivo magnetic imaging, electrode materials, and absorbents for heavy metal ion removal, and so on.^17–21 In catalysis, some successful examples are the development of iron-based nanoparticles which have proven to be highly active catalysts in heterogeneous catalytic oxidation, hydrogenation, and C–C bond coupling reactions, etc.^22–25 Like many other metal oxides, the properties of Fe₃O₄ NCs for use in catalytic field strongly depends on the surface structure of facets enclosing the crystals. Therefore, the preparation of Fe₃O₄ NCs with controllable size and shape along with the investigation of their catalytic properties continues to be of considerable interest. Up to now, despite Fe₃O₄ with various morphologies and particle sizes, including uniform nanocrystals, truncated cubes, nanocubes, truncated octahedra, nanooctahedra, and dodecahedra have been successfully synthesized by a rich variety of methods and devoted to optimize and enhance magnetic and electrochemical properties, etc.,^26–30 its shape-dependent applications in catalysis field are still scarce. To explore the nature of the structure–performance relationship of Fe₃O₄ NCs, the development of alternative methodologies for the facile and well-controlled synthesis of magnetic Fe₃O₄ NCs as well as understanding of the shape-dependent properties remains an important issue.³¹

Herein, we report a facile hydrothermal synthesis technique in a binary triethanolamine (TEOA)/water solvent system, and successfully obtain Fe₃O₄ nanocubes, octahedrons and polyhedrons. The shape of Fe₃O₄ NCs was strongly affected by the TEOA and NaOH concentration as well as the intrinsic surface energy of Fe₃O₄. The synthetic route is facile, controllable, and low-cost. More importantly, when used as catalyst for the catalytic reduction of p-nitrophenol into p-aminophenol with N₂H₄, the obtained (100) faceted Fe₃O₄ nanocubes exhibit a superior catalytic activity and cycle-to-cycle stability. Furthermore, several pivotal factors affected their catalytic activity were discussed.

Experimental section

Fabrication of Fe₃O₄ samples

All reagents used in the experiment were of analytical grade and used without further purification. A typical synthesis of cubic Fe₃O₄ nanocrystals was as following: 0.6 g of triethanolamine (TEOA) was dissolved into 18 mL of deionized water under stirring. Subsequently, 0.27 g of FeSO₄·7H₂O (1.0 mmol) and 2.0 mL of hydrazine hydrate (80%) then was added into the above solution. The mixture was stirred at room temperature to obtain an invisible green solution, followed by transferring the mixture into a 25 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 413 K for 12 h. After cooling to room temperature slowly, the resulted solid was collected by centrifugation and washed with distilled water and absolute ethanol to remove any ionic residual. The solid was dried in a vacuum oven at 343 K for 12 h and the yield of the product is approximately 95%. The octahedral Fe₃O₄ NCs were obtained by adding 5.0 g of triethanolamine and 0.2 g of NaOH to the hydrothermal reaction system in the same condition as the above. The bulk Fe₃O₄ particles used for comparison was prepared in the absence of TEOA and NaOH, while other conditions keep the same.

Characterization

The phase purity and crystal structure of the sample were characterized by X-ray power diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.541 87 Å). More details about the chemical structure and morphology of the sample were obtained from selected area electron diffraction (SAED) combined with transmission electron microscopy studies (HR TEM, Tecnai G2 F20 S-TWIN transmission electron microscope). Samples for TEM characterization were prepared by adding several drops of a solution of Cu NCs in TCE onto the 300 mesh copper grids with carbon support film. The morphologies and sizes of the samples were examined by field-emission scanning electron microscopy (FE SEM, Nova NanoSEM 230). The specific surface area of the samples was measured by nitrogen sorption at 77 K on a Micromeritics ASAP 2020 instrument and calculated by the Brunauer–Emmett–Teller (BET) method.

Catalytic performance

The catalytic reduction of p-nitrophenol to p-aminophenol by hydrazine hydrate in the presence of Fe₃O₄ catalysts was performed at 328 K in a glass reactor. In a typical catalytic test, a mixed solution of hydrazine (3.0 M) and NaOH (1.3 M) was prepared. Then, 4 mL mixture solution and 16 mL of p-nitrophenol solution (2 × 10⁻⁴ mol L⁻¹) were added successively into a glass reactor. Prior to the reaction, the system was filled with high-purity N₂ (>99.99%). This process was maintained ten minutes in order to completely remove O₂ in the system. After that, 20 mg of catalyst was dispersed into the above solution (pH ≈ 10.0) by gentle magnetic stirring. To investigate the effect of reaction temperature on the catalytic performance of the Fe₃O₄ sample, similar catalytic experiments were also performed at 298 K, 318 K and 328 K. Additionally, during the catalytic process, 2.0 mL of solution were taken at given time intervals and separated through centrifugation. The residual concentration of p-aminophenol in solution was analyzed by recording variations of the organics at the absorption band maximum in the UV-vis spectra using a Varian Cary 50 Scan UV-vis spectrophotometer. The normalized p-nitrophenol concentration can be expressed as: c/c₀ = A/A₀. In this equation, c and A is the residual p-nitrophenol concentration and measured absorption intensity at any reaction time, while c₀ and A₀ represent the initial p-nitrophenol concentration and absorption intensity, respectively. For the recycling experiment, the catalysts were collected, washed with deionized water, and reused in the next reaction.

Results and discussion

XRD patterns of the as-prepared Fe₃O₄ nanocubes and octahedrons were shown in Fig. 1. All the diffraction peaks of the samples can be entirely indexed to a pure face-centered cubic phase (fcc, space group Fd3̄m) of Fe₃O₄ (JCPDS card No. 19-0629) with the lattice constants a = b = c = 8.396 Å, and α = β = γ = 90°, which agree with the standard values of bulk Fe₃O₄. No characteristic peaks for other types of impurities such as FeO, Fe₂O₃, FeOOH, etc., were detected. Thus, it can be confirmed that both products are pure phase Fe₃O₄ particles and own a face-centered cubic inverse-spinel structure. In addition, it can be obviously seen that the diffraction peaks are strong and sharp, which suggests the good crystallinity of as-prepared Fe₃O₄ samples.

Fig. 1 Typical power XRD patterns of the as-prepared Fe₃O₄: (a) cubes and (b) octahedrons.

Fig. 2a shows a representative FESEM image of the Fe₃O₄ nanocubes sample prepared at 453 K with 0.6 g of TEOA. Very clearly, the sample is dominated by a morphology consisting of Fe₃O₄ with a cube-shaped nanostructure. According to the partially enlarged FESEM image shown in Fig. 2b, each nanocrystal maintains its individual cubic shape with a typical size of ca. 150 nm, and the surface of the cubic NCs is very smooth implying that there is no porous structure on their bodies. The chemical compositions of the as-obtained Fe₃O₄ sample were characterized by EDX. As shown in Fig. S1,† the peaks of Fe and O can be easily seen, and the atomic ratio of Fe/O is very close to the stoichiometry of Fe₃O₄ based on the calculation of peak areas. The impurity peaks, which are assigned to Au, come from the coating of powder in the SEM experiment. More details about the nanostructure and morphology of sample were done by TEM. It can be seen from Fig. 2c that the sample has cubic crystal morphology, which is consistent with the FESEM observations. Additionally, detailed crystal structures of the as-obtained Fe₃O₄ nanocubes were characterized using HRTEM. A typical high-resolution transmission electron microscopy (HRTEM) image of the Fe₃O₄ nanocube is shown in Fig. 2d. The distance between the adjacent lattice fringes is 0.21 nm, which can be assigned to the interplanar distance of the face-centered cubic Fe₃O₄ (100) plane. Meanwhile, it is clearly observed that the interfacial angle between the adjacent planes is 90° (Fig. 2b). Inset in part Fig. 2d, the electron diffraction (ED) pattern verifies that the as-prepared products show apparently single crystalline characteristics. These characteristic results suggest that the resulting Fe₃O₄ nanocubes are single crystals bounded by (100) planes.

Fig. 2 Typical (a) large-area and (b) partially enlarged FESEM images of the as-prepared Fe₃O₄ nanocubes. Typical (c) TEM and (d) HRTEM images of the as-obtained Fe₃O₄ nanocubes. Inset in part (d) is the corresponding SAED pattern.

Fig. 3a shows FESEM image of the Fe₃O₄ octahedrons prepared at 453 K by using 5.0 g of TEOA in the presence of NaOH. It can be seen that the sample is composed of a large quantity of monodisperse octahedral Fe₃O₄ particles. From partially enlarged FESEM images in Fig. 3b, it is further clearly seen that these regular Fe₃O₄ particles own an octahedron-shaped structure, and the facets of octahedrons are apparently distinguishable. It demonstrates well-defined shape of the Fe₃O₄ octahedron. The morphological characteristics of sample are also supported by TEM observation (Fig. 3c). More importantly, the yield and purity of the octahedral NCs are considerably high according to these images (Fig. 3a and c). To further obtain the detailed crystal structure of the octahedral Fe₃O₄ NCs, the high-resolution transmission electron microscopy (HRTEM) observations were carried out. Fig. 3d shows typical HRTEM images of the Fe₃O₄ octahedron. Among the image, a uniform lattice fringe and a spot pattern of the selected area electron diffraction (SAED) (insert in Fig. 4d) verify that the Fe₃O₄ sample show apparently single crystalline characteristic. The distance between the adjacent lattice fringes is 0.482 nm, which can be assigned to the interplanar distance of the Fe₃O₄ (111) plane. Moreover, from Fig. 3b, it is found that the interfacial angle between the adjacent planes is 60°. Accordingly, the resulting octahedron-shaped Fe₃O₄ sample is single crystals bounded by (111) planes.

Fig. 3 Typical (a) large-area and (b) partially enlarged FESEM images of the as-prepared Fe₃O₄ octahedrons. Typical (c) TEM and (d) HRTEM images of the as-obtained Fe₃O₄ octahedrons. Inset in part (d) is the corresponding SAED pattern.

Fig. 4 FE-SEM images of the products obtained at 140 °C with different solvothermal treatment time: (a) 3 h, (b) 6 h, (c) 9 h, (d) 12 h, (e) 18 h, and (f) 24 h.

In order to understand the formation process of the Fe₃O₄ NCs with various morphologies and the possible growth mechanism, effects of the TEOA and NaOH usage and the solvothermal treatment time were investigated by FESEM analysis. It is found that the unique organic basic environment provided by TEOA and NaOH usage is essential for the formation of the regular Fe₃O₄ nanocubes and octahedrons, while the solvothermal treatment time also plays a vital role in the formation of Fe₃O₄ nanocubes and polyhedrons. When without TEOA or ethanolamine and diethanolamine instead of TEOA were used, the resulting product is not the regular Fe₃O₄ nanocubes but only a large number of irregular Fe₃O₄ particles and few Fe₃O₄ cubes (see Fig. S2 of ESI†). Additionally, the SEM analysis shows that formation of Fe₃O₄ nanocubes also needs an optimal solvothermal treatment time under present hydrothermal synthetic conditions. At an early stage of reaction, the primary Fe₃O₄ nuclei were formed and the renascent nuclei grew into cube-like nanocrystals driven by the adsorption effect of TEOA (Fig. 4a and b). Increasing hydrothermal time from 6 h to 9 h, the regular cube-shaped Fe₃O₄ NCs gradually formed (Fig. 4c and d). As the reaction went on further, the Fe₃O₄ nanocubes gradually aggregated together and had the trend to form polyhedrons, as shown in Fig. 4e and f. This could be because the as-obtained Fe₃O₄ nanocube aggregates underwent a dissolution–recrystallization process. The progress of shape change usually accompanies the size and intrinsic surface energy variation.³¹ These results show that the Fe₃O₄ shape evolution from cube shape to polyhedron shape is strongly affected by TEOA and solvothermal treatment time.

Nevertheless, NaOH is an obligatory assistant agent for the morphology transition from Fe₃O₄ cube shape to octahedron shape apart from TEOA and solvothermal treatment time. Herein, we observed the influence of the NaOH usage on the shape evolution for SEM, as shown in Fig. S3.† When no NaOH is added, the resulting solid is mainly small diameter Fe₃O₄ nanocubes in the absence of TEOA (see Fig. S3a of ESI†). Note that NaOH amount is increased to 0.2 g, the product is composed of a large number of Fe₃O₄ octahedrons (see Fig. S3b of ESI†). Increasing the amount of the NaOH from 0.2 g to 0.4 g, the irregular Fe₃O₄ particles are obtained (see Fig. S3c of ESI†). These results indicate that NaOH is responsible for the morphology transition from cube shape to octahedron shape. It might be pH that affects the formation of cubic and octahedral nanocrystals, because the nucleation rate of Fe₃O₄ was influenced by different pH, that is to say, the reaction rate was changed, and so it might affect the crystal growth. For an fcc crystal, the surface energies corresponding to different crystallographic facets are in the order of {111} < {100} ≪ {110}.¹⁶ Generally, during the crystal growth processes under equilibrium conditions, the high-energy facets diminish quickly and the crystals spontaneously evolve into a specific shape with the lowest surface energy exposed facets to minimize the total surface free energy.³² Obviously, in our work, the amount of TEOA and NaOH adjusts the relatively growth rate of the 〈100〉 and 〈111〉, thus changes the morphology from cube shape to octahedron shape.

The catalytic hydrogenation of p-nitrophenol to its corresponding p-aminophenol as a model reaction was used to evaluate the catalytic performance of the as-obtained Fe₃O₄ NCs with the assistance of N₂H₄. The controlled blank reaction in the absence of any catalyst showed no p-aminophenol formation. For comparison, the catalytic activity of the bulk Fe₃O₄ particles was also determined. Rate of catalytic reduction of p-nitrophenol as a function of reaction time are shown in Fig. 5d. Upon the addition of N₂H₄ and NaOH, the color of the solution turned to bright yellow, meanwhile, an absorption band appears at ∼400 nm (see Fig. S4 of ESI†). This means that the p-nitrophenolate ions become the dominant species due to the alkalinity of the solution.²⁷ According to the UV-vis spectra, for all Fe₃O₄ samples, after these catalysts were introduced into the p-nitrophenol/N₂H₄/NaOH systems, the intensity of the absorption bands at 400 nm reduced gradually with the increase of reaction time (Fig. 5a–c). At the same time, new absorption bands at ca. 300 nm appeared as shoulders, which are ascribed to the formation of p-aminophenol.³³ One isosbestic points was located at ca. 300 nm, indicating that only one type of product was formed during the reaction. According to our experimental results, only 19.7% of p-nitrophenol was reduced after 18 min reaction in the presence of bulk Fe₃O₄ particles (Fig. 5a). Additionally, 22.4% of p-nitrophenol was eliminated from the system with Fe₃O₄ octahedrons bounded by (111) planes (Fig. 5b). In contrast, almost 100% of p-nitrophenol was reduced on cubic Fe₃O₄ NCs bounded by (100) planes in the same reaction time (Fig. 5c). The results reveal that the Fe₃O₄ nanocubes show the highest the activity, and the catalytic hydrogenation rate is decreased in the order Fe₃O₄ nanocubes > Fe₃O₄ octahedrons > bulk Fe₃O₄ particles (Fig. 5d). Additionally, successive optical photographs of the reaction solution by reducing p-nitrophenol into p-aminophenol over Fe₃O₄ nanocubes are shown in Fig. 6. It can be seen that increasing reaction time from 0 min to 21 min, the color of the solution finally turned to white, indicating catalytic hydrogenation of p-nitrophenol is completed.

Fig. 5 Time-dependent UV-vis absorption spectra of the p-nitrophenol solution in the presence of (a) bulk Fe₃O₄ particles, (b) Fe₃O₄ octahedrons, and (c) Fe₃O₄ nanocubes; (d) rate of catalytic reduction of p-nitrophenol solution over bulk Fe₃O₄ particles, Fe₃O₄ octahedrons, and Fe₃O₄ nanocubes as a function of traction time in the presence of N₂H₄. Catalyst, 20 mg; volume of solution, 20 mL; reaction temperature, 318 K.

Fig. 6 Successive optical photographs of the conversion from p-nitrophenol to p-aminophenol with Fe₃O₄ nanocubes at different reaction times.

To reveal the origin and activity of Fe₃O₄ NCs for the reduction of primary p-nitrophenol to the corresponding p-aminophenol by an excess of N₂H₄/NaOH without the assistance of noble metals, the kinetic analysis for catalytic reduction is necessary. Here, the reduction of p-nitrophenol to p-aminophenol was used as a model reaction to obtain some pivotal kinetic parameters. Fig. 7a shows the p-nitrophenol conversion as a function of reaction temperature. The results show that as the reaction temperature increases from 298 to 318 K, the p-nitrophenol conversion increases gradually from 7.5% to 88% after a reaction time of 12 min. The optimal temperature may be usable for analysis of the reaction kinetics from 298 to 318 K. As can be observed in Fig. 7a, reduction rate of p-nitrophenol increases with an increase in the temperature. This observation is understandable because the reducibility of N₂H₄ is enhanced by increasing the temperature. Generally speaking, the apparent activation energy (E_a) is a very important parameter of a catalyst, which can be applied to evaluate its catalytic performance. In this study, according to the Arrhenius equation of ln k = ln A − E_a/RT, we plotted ln k versus 1/T, where k is the rate constant obtained by linearly fitting the conversion of p-nitrophenol within the initial 12 min of the reaction at an indicated temperature. The plot of ln k versus 1/T is satisfactorily linear, as shown in Fig. 7b. The slope and intercept are ∼1 × 10⁻⁵ and 35.2, respectively. The apparent activation energy (E_a) and pre-exponential factor (A) calculated by the Arrhenius equation is 100 ± 5.8 kJ mol⁻¹.

Fig. 7 (a) p-Nitrophenol conversion profiles obtained at the indicated reaction temperature using Fe₃O₄ nanocubes catalyst. (b) Arrhenius plot of the reduction of p-nitrophenol with hydrazine hydrate. Catalyst, 20 mg; volume of solution, 20 mL.

However, we also observed, in this study, when no NaOH was introduced to the reaction system, while other reaction conditions were maintained constant, the catalytic hydrogenation rate of the Fe₃O₄ nanocubes is slightly less than that of the same Fe₃O₄ catalyst in presence of NaOH. This means that NaOH is favorable for p-nitrophenol reduction towards p-aminophenol. NaOH-mediated alkalescent solution facilitates the formation of p-nitrophenolate,³⁴ which enhances the donating ability of donors in p-nitrophenolate. In theory, the structure is unfavourable for the catalytic hydrogenation of p-nitrophenol. However, we should point out that introduction of NaOH led to decrease in the oxidation potential of N₂H₄,³⁵ which enhances reducibility of hydrazine hydrate. Therefore, catalytic hydrogenation reaction in p-nitrophenol reduction may be facilely completed in presence of Fe₃O₄ nanocubes.

In addition, reusability and stability are both important concerns for a catalyst. In order to investigate the durability of the catalyst systems, Fe₃O₄ nanocubes were employed as a representative system. After each run, the Fe₃O₄ nanocubes catalyst was evacuated for 18 min and was reused in the next run. As illustrated in Fig. 8, when the same catalyst was utilized up to five successive cycles, the plots of c/c₀ versus time for the reduction of p-nitrophenol remained almost unchanged. Namely, Fe₃O₄ nanocubes retain ca. 96% of their original catalytic activity after 5 cycles, indicating that the as-prepared Fe₃O₄ nanocubes catalyst exhibits good reusability and stability.

Fig. 8 The reusability of the as-prepared Fe₃O₄ nanocubes for the reduction of p-nitrophenol to p-aminophenol with N₂H₄. Catalyst, 20 mg; volume of solution, 20 mL; reaction temperature, 318 K.

Based on the precedent structural characterizations, although the Fe₃O₄ samples possessed the same phase structure (see Fig. 1 and S3, in the ESI†), the Fe₃O₄ nanocubes exhibited best catalytic hydrogenation activity in p-nitrophenol reduction reaction. The observed enhancement of the catalytic activity over the Fe₃O₄ nanocubes could be ascribed to the following three aspects. Firstly, we believed that the enhanced catalytic activity on Fe₃O₄ nanocubes is mainly dependent on their surface structures in present system. It is well-known that the surface structure of catalyst plays vital roles to their catalytic activities in chemical reactions because the catalytic reaction takes place on the surface. In the case of Fe₃O₄ NCs of different shapes, the variation in shape implies that Fe atoms can be arranged in various forms. In our cases, the surface of Fe₃O₄ catalyst can be dominated by {111} and {100} facets that determine the level of catalytic activity. Studies have shown that the planes with higher surface energy are more reactive. Therefore, the Fe₃O₄ nanocubes enclosed by the less stable {100} planes exhibited the higher catalytic activity in the hydrogenation of p-nitrophenol. Some studies^36,37 have also demonstrated that the catalysts exposed with various reactive facets or directions have an important effect on the enhanced catalytic activities. Secondly, among Fe₃O₄ samples, it is noteworthy that BET surface area of the Fe₃O₄ nanocubes is 11.8 m² g⁻¹, which is slightly greater than that of the Fe₃O₄ octahedrons (8.2 m² g⁻¹) and of the bulk Fe₃O₄ particles (8.5 m² g⁻¹), correspondingly, the catalytic hydrogenation rate is decreased in the order Fe₃O₄ cubes > Fe₃O₄ octahedrons > bulk Fe₃O₄ particles (Fig. 5). The result indicates that the enhanced catalytic activity of Fe₃O₄ nanocubes can be also related to the specific surface area to a certain extent. This might be because high-specific-surface-area Fe₃O₄ nanocubes can provide more reaction sites, therefore, improve their catalytic activities. However, we also observed that catalytic hydrogenation rates of the Fe₃O₄ samples in the order are not completely consistent with values of surface area of corresponding samples, and the result suggests that the difference in the specific surface area is not mainly factor leading to the difference in the catalytic hydrogenation activity. In addition, on the basis of preceding experimental result, in our cases, the unique alkaline basic environment provided by hydrazine hydrate and NaOH is also an essential factor for the catalytic hydrogenation of p-nitrophenol to its corresponding p-aminophenol in presence of Fe₃O₄ sample. Generally, electrode potentials of hydrazine hydrate decreases with an increase in the solution alkalinity to a certain extent. This means that the sodium hydroxide-mediated alkalescent medium can greatly enhances reducibility of hydrazine hydrate in reaction system, and thus accelerate the reaction rate of the catalytic hydrogenation of p-nitrophenol over Fe₃O₄ nanocubes. In view of above results analysis, our work suggests that the enhanced catalytic activity of the Fe₃O₄ nanocubes can be a result of the synergistic effect of the beneficial surface structures, optimized basic reaction system, and high special surface area for catalytic reduction of p-nitrophenol to its corresponding p-aminophenol in the presence of N₂H₄.

Conclusions

In summary, high-quality Fe₃O₄ nanocubes and octahedrons were successfully synthesized via a simple and general hydrothermal synthetic route in a binary TEOA/H₂O solution system. The experimental results demonstrated that the Fe₃O₄ nanocubes were dominated by (100) planes. In contrast, addition of increasing amounts of NaOH, and it adjusts the relatively growth rate of the (100) and (111), thus changes the morphology from cube shape to octahedron shape. The detailed growth mechanism of these Fe₃O₄ NCs was suggested. The catalytic investigations show that the well-formed Fe₃O₄ nanocubes exhibit superior catalytic hydrogenation activities and good cycling stability. Our results also indicated that the geometric shape and particle size of the Fe₃O₄ NCs may be an important factor in the catalyst design besides optimized reaction system for reducing p-nitrophenol into p-aminophenol with N₂H₄. The synthetic strategy reported here will be not only helpful in systematically exploring fabrication of Fe₃O₄ micro-/nanocrystals with reactive facets but it also provides a feasible approach for developing highly light-active catalysts for their catalytic application.

Acknowledgements

This work was financially supported by the Natural Natural Science Foundation of China (Grant no. 21361019 and 21463019).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S5. See DOI: 10.1039/c6ra03770d

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