Grass-like Co₂P superstructures: direct synthesis between elements, forming mechanism and catalytic properties

Abstract

In this paper, a two-step solution process was designed for the preparation of grass-like Co₂P superstructures with enhanced catalytic activity. Firstly, cobalt microspheres were solvothermally fabricated through the reaction between N₂H₄·H₂O and CoSO₄·7H₂O in glycol at 100 °C for 10 h. Then, freshly-prepared Co microspheres reacted with white phosphorus via a hydrothermal route at 180 °C for 16 h to produce grass-like Co₂P superstructures. The phase and the morphology of the as-prepared products were characterized by powder X-ray diffraction (XRD), energy dispersive spectrometry (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Experiments showed that the as-prepared grass-like Co₂P superstructures presented outstanding catalytic activity in the reduction of some aromatic nitro compounds such as 4-nitrophenol, 2-nitrophenol and 4-nitroaniline in excess NaBH₄ solution. Simultaneously, several organic dyes including Rhodamine B, Pyronine B and Safranine T could be also rapidly reduced in excess NaBH₄ solution, which provides a new choice for the room-temperature decoloration of the dyes.

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1. Introduction

Over the past decade, transition-metal phosphides have been drawing extensive research interest due to their enriching compositions and potential applications in the photocatalytic degradation of organic pollutes, the removal of heavy metal ions, Li-ion batteries and supercapacitors.^1–5 More importantly, transition-metal phosphides with highly active hydro-desulfurization (HDS), hydrodenitrogenation (HDN) properties have become new choices as promising candidates of the next-generation catalysts.^6–8 As one of the important members in the transition-metal phosphide family, phosphides of cobalt have been extensively investigated owing to their interesting magnetic, photocatalytic, catalytic and anode material properties.⁹

Usually, phosphides of cobalt include Co₂P, CoP, CoP₂ and CoP₃.¹⁰ Among them, Co₂P is the most common product. To obtain Co₂P nanostructures, many methods have been developed, including the traditional gas–solid route, solid phase reaction, and solution phase synthesis under high temperature, and mild solvothermal/hydrothermal preparation.^11–22 Employing the above technologies, Co₂P nanostructures with various morphologies have been successfully obtained such as nanorods,¹¹ nanotubes and nanospheres,¹² nanowires,¹³ urchin-like nanocrystals,¹ and nanoflowers.^14,15 For example, Peng and coworkers¹⁶ fabricated Co₂P nanostructures through the solid-phase reaction between NaH₂PO₂·H₂O and CoCl₂·6H₂O at 600 °C for 6 h. Hyperbranched Co₂P nano-structures were synthesized via decomposing cobalt oleate in trioctylphosphine oxide (TOPO) at 350 °C;¹⁷ or highly toxic phosphines (e.g. PH₃) gas or phosphorus pentachloride vapor reacted with metals or metal salts at high temperature for the preparation of transition-metal phosphides including Co₂P.¹⁸ In the abovementioned routes to produce Co₂P nanostructures, obviously, the high temperature above 350 °C is necessary. Simultaneously, some toxic and/or expensive organic compounds are selected, which is unfavorable for environmental protection and energy saving. Thus, mild solvothermal/hydrothermal technology was developed.^19–22 However, it is still a huge challenge to material scientists developing a facile and feasible method to prepare morphology- and phase-controlled cobalt phosphides nanocrystals.

Recently, our group reported a direct element-reaction route to successfully prepare spherical Ni_xP_y hollow superstructures and investigated the conversion process from metal nickel to phosphides of nickel.²³ The Kirkendall effect was used for explaining the formation of spherical hollow superstructures. Moreover, experiments also uncovered that the catalytic activity of the product for the reduction of 4-nitrophenol was enhanced in turn from metal Ni, to Ni@Ni_xP_y and final to Ni_xP_y.²³ Considering the similar properties between cobalt and nickel, in the current work, we attempted to synthesize phosiphides of cobalt nanostructures via the above direct element-reaction. However, different experimental phenomena were found: firstly, only Co₂P phase was obtained; secondly, no hollow core–shell structures were observed in the formation of Co₂P phase; finally, the product presented grass-like superstructures constructed by abundant nanorods. Furthermore, the as-obtained grass-like Co₂P superstructures still exhibited the excellent catalytic activity for the reduction of organic dyes including Rhodamine B (RhB), Pyronine B (PB) and Safranine T (ST) in NaBH₄ aqueous solution besides the catalytic reduction of aromatic nitro compounds. Compared with previous methods to prepare Co₂P nanostructures, the present direct element-reaction route is not found in the literature.

2. Experimental

All reagents and chemicals are analytically pure, bought from Shanghai Chemical Company and used without further purification.

2.1 Synthesis of Co microspheres

In a typical experiment procedure, 1 mmol CoSO₄·7H₂O (0.2813 g), 2 mmol NaOH (0.08 g) and 5 mL hydrazine hydrate (80%) were dissolved into 25 mL of ethylene glycol under vigorous magnetic stirring at room temperature. Subsequently, the as-obtained mixed solution was transferred into a Teflon-lined stainless steel autoclave with 40 mL capacity. The reaction was carried out in an oven at 100 °C for 10 h. After the system was cooled to room temperature naturally, the black product with the yield of ∼93.4% was gathered by a magnet, washed with distilled water and ethanol several times, and dried in vacuum at 60 °C for 4 h.

2.2 Synthesis of grass-like Co₂P superstructures

To prepare Co₂P, 50 mg Co microspheres were firstly dispersed into 30 mL deionized water under ultrasonication. After the above suspension was transferred into a Teflon-lined stainless steel autoclave of 40 mL capacity, 0.1 g white phosphorus (WP) was added. It is noteworthy that all operations relating to WP were carried out in water. The autoclave was sealed and maintained at 180 °C for 16 h, then allowed to cool down to room temperature naturally. The precipitate was still black, but no magnetism was detected, implying the production of cobalt phosphides. The black precipitate with the yield of ∼53.5% was collected, washed with distilled water and absolute ethanol several times to remove impurities, and dried in vacuum at 60 °C for 6 h.

2.3 Characterization

The X-ray powder diffraction patterns of the products were carried out on a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu Kα radiation (λ = 0.154060 nm), employing a scanning rate of 0.02° s⁻¹ and 2θ ranges from 30° to 80°. Transmission electron microscopy (TEM) images were carried out on a FEI Tecnai G² 20 transmission electron microscope, employing an accelerating voltage of 200 kV. SEM images and EDS analysis of the product were obtained on a Hitachi S-4800 field emission scanning electron microscope, employing an accelerating voltage of 5 kV and 15 kV, respectively. The content of Co in the final product was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 5300DV-ICP, Perkin-Elmer).

2.4 Performance tests

2.4.1 Catalytic reduction of organic dyes molecules. To investigate the catalytic ability of the as-obtained product for the reduction of organic dye molecules like RhB (C₂₈H₃₁N₂O₃Cl), PB (C₂₁H₂₇ClN₂O), and ST (C₂₀H₁₉ClN₄) in NaBH₄ solution, 30 mg L⁻¹ of solution with various dyes was separately prepared. In a typical experiment, 1 mL of dye solution was mixed with 1 mL of NaBH₄ solution. Then, 1 mL of Co₂P suspension was added. The total volume of the mixed system was kept at 3 mL. Here, 10 mg L⁻¹ of the dye solution was obtained. The corresponding molar concentrations were 2.09 × 10⁻⁵ mol L⁻¹ (RhB), 2.79 × 10⁻⁵ mol L⁻¹ (PB) and 2.85 × 10⁻⁵ mol L⁻¹ (ST), respectively. The ones of NaBH₄ and catalyst were 2.0 × 10⁻² mol L⁻¹ and 2.24 × 10⁻⁴ mol L⁻¹ in turn. The reduction process was monitored with a Metash 6100 UV-vis spectrophotometer.

As a control, a blank experiment was carried out, too. 10 mL 6.72 × 10⁻⁴ mol L⁻¹ of Co₂P were firstly dispersed in 20 mL organic dyes solution with a similar concentration under the ultrasonic assistance. Then, the mixed system was stirred in the dark for another 30 min to ensure a sorption–desorption equilibrium.

2.4.2 Catalytic reduction of nitro compounds. Similarly, the catalytic reduction of some aromatic nitro compounds in NaBH₄ solution was also investigated, including 4-nitrophenol (4-NP), 2-nitrophenol (2-NP) and 4-nitroaniline (4-NA). Here, the concentrations of 4-NP, NaBH₄ and catalyst were in turn 1.0 × 10⁻⁴ mol L⁻¹, 2 × 10⁻² mol L⁻¹ and 6.72 × 10⁻⁴ mol L⁻¹.

3. Results and discussion

3.1 Morphology and structure characterization

It is well known that ethylene glycol (EG) molecules contain OH groups, which have stronger coordinative ability to some metal atoms. Thus, EG molecules are easily adsorbed at the surroundings of metal particles in the system. The presence of EG molecules on the particle surfaces can efficiently reduce the agglomeration of the product.²⁴ In the present work, it was favourable to form monodispersive Co microspheres to select EG as the solvent. Fig. 1a depicts a typical SEM image of the magnetic product prepared by the solvothermal route at 100 °C for 10 h. A great deal of microspheres with the diameter range from 0.5–1.5 μm is clearly visible. The XRD pattern of the magnetic product is shown in Fig. 1b, from which four obvious diffraction peaks centered at 41.6°, 44.2°, 47.5° and 75.9° can be seen. By comparison with the PDF card files no. 89-4308 and no. 89-4307, the as-obtained magnetic matter should be a mixed phase of hexagonal and cubic Co.

Fig. 1 (a) SEM image and (b) XRD pattern of the magnetic matter prepared by the solvothermal route at 100 °C for 10 h.

After the as-obtained Co microspheres reacted with WP under the hydrothermal conditions at 180 °C for 16 h, however, the product lost the magnetism, implying the formation of a new matter. The XRD analysis shown in Fig. 2a proved that the final product belonged to orthorhombic Co₂P. To confirm the formation of Co₂P, the EDS and ICP-AES technologies were employed. Fig. 2b exhibits the EDS analysis of the as-prepared product. The strong peaks of Co and P can be easily seen besides the weak C and O peaks. Based on the calculation of peak areas, the atomic ratio of Co/P in the final product is 1.92/1, which is very close to the stoichiometry of Co₂P. The weak C and O peaks should be attributed to the physical adsorption of the sample to carbon and oxygen species in air. Furthermore, the ICP-AES analyses showed that the content of Co was ∼65% in the final product, close to 66.7% in Co₂P.

Fig. 2 (a) The XRD pattern and (b) the EDS analysis of the final product prepared under the hydrothermal conditions at 180 °C for 16 h.

Simultaneously, compared with magnetic Co microspheres the final product presented markedly different morphology. As shown in Fig. 3a, grass-like superstructures are clearly visible. High magnification SEM observations discovered that these superstructures were constructed by abundant nanorods with sharp tips (see Fig. 3b). The above result was also proven by TEM observations. As described in Fig. 3c, some nanorods with sharp tips can be readily seen.

Fig. 3 Electron micrographs of the final product prepared under the hydrothermal conditions at 180 °C for 16 h: (a) low magnification, (b) high magnification SEM and (c) TEM image.

3.2 Possible formation mechanism of Co₂P

To ascertain the formation process of grass-like Co₂P superstructures under the current hydrothermal conditions, a time-dependent shape evolution experiment was designed. As shown in Fig. 4a, the product still presented spherical structures after reacting for 2 h. However, some pores appeared in the surfaces of many microspheres, implying that microspheres were partially etched. Although the product could be moved by a magnet, XRD analysis showed very weak diffraction peaks of metal cobalt (see Fig. 5a). When the reaction was carried out at 180 °C for 4 h, some pink matters appeared. SEM observations showed that small nanoparticles markedly increased in the product besides porous microspheres (see Fig. 4b). Here, XRD analysis uncovered that the diffraction peaks belonged to hexagonal Co₁₁(HPO₃)₈(OH)₆ were detected (see Fig. 5b). Upon further prolonging the reaction time to 8 h, the pink matter was the main product, which was still hexagonal Co₁₁(HPO₃)₈(OH)₆ (see Fig. 5c). SEM observations showed that semispherical products constructed by nanorods were produced (see Fig. 4c). Continuously prolonging the reaction time to 12 h, the black product appeared again, but no magnetism was detected. The final product still presented semispherical structures (see Fig. 4d).

Fig. 4 SEM images of the products prepared by the hydrothermal route at 180 °C for different durations: (a) 2 h, (b) 4 h, (c) 8 h and (d) 12 h.

Fig. 5 XRD pattern of the products prepared by the hydrothermal route at 180 °C for different durations: (a) 2 h, (b) 4 h, (c) 8 h and (d) 12 h.

The XRD analysis showed that only diffraction peaks of Co₂P were found (see Fig. 5d). After 16 h, grass-like Co₂P superstructures were successfully formed (see Fig. 3). In the above formation process of grass-like Co₂P superstructures, obviously, no core–shell structures were detected, which is different from the formation of Ni_xP_y.²³ Thus, it is possible that the generation mechanism of Co₂P differs from that of Ni_xP_y, which was formed by the direct reaction between Ni microspheres and WP.²³ To investigate the influence of water on the formation of Co₂P, we used ethanol as the solvent instead of water. Under the same experimental conditions, the product still presented spherical microstructures (see Fig. 6a); and the XRD analysis proved that cobalt was still the main product (see Fig. 6b). This indicated that water played an important role in the generation of Co₂P.

Fig. 6 (a) SEM image and (b) XRD pattern of the product prepared from the ethanol system under the same experimental conditions.

In the above time evolution experiments, moreover, the pH value of the system also presented marked change. The pH was ∼7.0 before reaction. After reacting for 2 h, the pH decreased to 5.1. After 4 h, the pH increased to 6.8 again. However, when the reaction time was prolonged to 8 h, the pH dramatically reduced to ∼2.0. After 12 h, the pH of the system was ∼1.2. It well known that WP can react with water molecules under the enhanced temperature to produce H₃PO₃ and PH₃.^25,26 Thus, the pH decreased in the initial stage. Subsequently, the produced H₃PO₃ was consumed since it reacted with Co microspheres to form pink Co₁₁(HPO₃)₈(OH)₆, which caused the rise of the pH. Simultaneously, the surfaces of Co microspheres became rough; and some pores were formed (see Fig. 4a and b). With the extension of the reaction time to 8 h, pink Co₁₁(HPO₃)₈(OH)₆ gradually became the main product. Meanwhile, due to the precipitation–dissolution equilibrium of Co₁₁(HPO₃)₈(OH)₆ few free Co²⁺ ions existed in water, which reacted with PH₃ to produce black Co₂P and released H⁺ ions. This led to the dramatic pH decrease of the system. The increased acidity further promoted the dissolution of Co₁₁(HPO₃)₈(OH)₆.²⁷ After reacting for 12 h, hence, the final product became black Co₂P. The related reactions are simply described as follows:

P₄ + 6H₂O → 2H₃PO₃ + 2PH₃ (1)

11Co + 8H₃PO₃ + 6H₂O → Co₁₁(HPO₃)₈(OH)₆ + 11H₂ (2)

Co₁₁(HPO₃)₈(OH)₆ ⇋ 11Co²⁺ + 8HPO₃⁻ + 6OH⁻ (3)

Co²⁺ + PH₃ → Co₂P + H⁺ (4)

3.3 The catalytic property

3.3.1 Catalytic reduction of organic dye molecules. Recently, Kundu et al. reported a NaBH₄-reduction route for the decoloration of several dye molecules under the assistance of CoO nanorods.²⁸ In the present work, we employed grass-like Co₂P superstructures as the catalyst to study the decoloration of several dyes, RhB, PB and ST, in excess NaBH₄ aqueous solution without the irradiation of UV or visible light. In the UV-vis absorption spectra of three dyes, the strongest absorption peaks separately locate at 552 nm for PhB, 527 nm for PB and 518 nm for ST. To investigate the interaction between the catalyst and dye molecules, only catalyst was introduced into the above dye solutions and stirred in the dark for 30 min. As shown in Fig. S1,† the absorption peak intensities of three dyes only slightly decreased, which should be attributed to the adsorption of the catalyst. Namely, no reaction took place between the catalyst and dye molecules. Similarly, when only NaBH₄ was added into the dye solutions the absorption peak sites and intensity of three dyes did not change, indicating that no reductive reactions took place between the dye and NaBH₄, too. After grass-like Co₂P superstructures were introduced the above dye-NaBH₄ systems, however, the peak intensities decreased with the extension of the reaction time. Fig. 7a–c show the UV-vis absorption spectra of dye-NaBH₄ systems in the presence of 2.24 × 10⁻⁴ mol L⁻¹ grass-like Co₂P superstructures for different durations. It is clearly visible that the peak intensities change with the time. Nevertheless, three dyes present different change trends of the peak intensities (see Fig. 7d). Under the catalysis of the same Co₂P amount, the concentration of RhB almost linearly decreases; the one of PB dramatically reduces at the initial stage and then, slowly decreases; while ST is only reduced slightly before 25 min, and subsequently ST is rapidly decolored within 5 min, implying the presence of an induction period. Since the reductive experiments were carried out under the same conditions, the above different change trends should be ascribed to the molecule structures of three dyes. Kundu et al. considered that the dye molecules accepted electrons transferred from BH₄⁻ ions to cause the reduction of dyes. This electron transfer took place via the catalyst (here is Co₂P) to the dye molecules.²⁸ In three dye molecules, oxygen atom exists in RdB/PB, and nitrogen atom in ST. Since O owns bigger electronegativity than N, the electron density of O atom in RdB/PB molecule is bigger than that of N atom in ST, too. Hence, RdB and PB are more easily adsorbed at the surrounding of the catalyst than ST. Thus, the molecules of different dyes display different affinity to Co₂P catalyst, which causes the rate difference of the electron transfer for each dye. Finally, three dyes exhibit different reductive rates.

Fig. 7 UV-Vis absorption spectra of Rhodamine B (a), Pyronine B (b) and Safranine T (c) in NaBH₄ solution with the presence of 2.24 × 10⁻⁴ mol L⁻¹ of grass-like Co₂P superstructures. (d) The concentration–time curves of three dyes.

3.3.2 Catalytic reduction of nitrobenzene compounds. Furthermore, the as-obtained grass-like Co₂P superstructures also presented good catalytic activity for the reduction of some aromatic nitrocompounds in excess NaBH₄ solution, including 4-nitrophenol (4-NP), 4-nitroaniline (4-NA) and 2-nitrophenol (2-NP). Fig. 8a–c separately exhibits the typical UV-vis absorption spectra of three aromatic nitro compounds taken at different reaction durations in the presence of 6.72 × 10⁻⁴ mol L⁻¹ Co₂P catalysts. As shown in Fig. 8a, after the Co₂P catalysts were introduced into 4-NP–NaBH₄ system, the intensity of the absorption peak at 400 nm obviously decreased since the reductive reaction proceeded. Meanwhile, a new absorption peak at ∼307 nm appeared which is the characteristic absorption peak of 4-AP.²⁹ After 8 min, the peak at 400 nm almost disappears, indicating the completion of the reductive reaction.

Fig. 8 UV-vis absorption spectra of various reduction systems taken at different reaction durations in the presence of 100 mg L⁻¹ Co₂P catalysts: (a) 4-NP, (b) 4-NA and (c) 2-NP. (d) The change curves of the concentration of aromatic nitro compound with the reaction time in the presences of Co₂P catalysts.

Similarly, the absorption peak intensities of 4-NA–NaBH₄ system and 2-AP–NaBH₄ system also decreased with the reactive time after the Co₂P catalysts were introduced, respectively (see Fig. 8b and c). It is noteworthy that no inducing period exists during the reduction of 4-NP. With the consumption of reactants, the reaction rate gradually slowed down. During the reductions of 4-NA and 2-NP, however, both peaks slowly decreased within 0–2 min (for 4-NA) and 0–3 min (for 2-NP), respectively, implying that the reactions were at the inducing stages; then, the peak intensities rapidly decreased, indicating the burst of the reaction. Fig. 8d depicts the change curves of the concentration of aromatic nitro compound with the reaction time in the presences of Co₂P catalysts. It can be readily found that three curves are markedly different. Since only aromatic nitro compounds are different in three reductive systems, the above different change curves should also be attributed to their different molecular structures.

Moreover, the amount of the Co₂P catalyst could tune the reductive reaction rate of aromatic nitro compounds. As a case, the reductive reaction of 4-NP to 4-AP was selected. As shown in Fig. 9, when the amount of the catalyst was 1.34 × 10⁻⁴, 2.68 × 10⁻⁴, 4.7 × 10⁻⁴ and 6.72 × 10⁻⁴ mol L⁻¹, it took 14, 10, 7 and 6 min for the reduction of 90% 4-NP, respectively. Since the reductive rate of 4-NP was only slightly promoted with the concentration increase of the catalyst from 4.7 × 10⁻⁴ to 6.72 × 10⁻⁴ mol L⁻¹, the catalyst concentration of 4.7 × 10⁻⁴ mol L⁻¹ should be optimum in the current experiments.

Fig. 9 The influence of the original amount of the catalyst on the reduction of 4-NP.

4. Conclusions

In summary, grass-like Co₂P superstructures have been successfully prepared via a simple hydrothermal route employing freshly-prepared Co microspheres and WP as starting reactants in the absence of any additive or template. Experiments showed that the formation of Co₂P underwent a complex process, including the dismutation of WP to produce H₃PO₃ and PH₃, the activation of Co microspheres by H₃PO₃, and the reaction of Co²⁺ and PH₃. It was found that the as-prepared grass-like Co₂P superstructures presented outstanding catalytic activity in the reduction of several organic dyes including Rhodamine B, Pyronine B and Safranine T and some aromatic nitro compounds such as 4-nitrophenol, 2-nitrophenol and 4-nitroaniline in excess NaBH₄ solution. This provides not only a new selection of catalyst for the catalytic reduction of aromatic nitro compounds in aqueous solution, but also an alternative route for the room-temperature of the dyes in the absence of outer light sources.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21171005 and 21571005) for the fund support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra21579j

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