Synthesis of a Ni2P/Ni12P5 bi-phase nanocomposite for the efficient catalytic reduction of 4-nitrophenol based on the unique n–n heterojunction effects

Feng-Yu Tian , Dongfang Hou *, Wei-Min Zhang , Xiu-Qing Qiao and Dong-Sheng Li *
College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key laboratory of inorganic nonmetallic crystalline and energy conversion materials, China Three Gorges University, Yichang 443002, P. R. China. E-mail: dfhouok@126.com; lidongsheng1@126.com

Received 1st July 2017 , Accepted 14th July 2017

First published on 14th July 2017


A novel heterostructure catalyst of Ni2P/Ni12P5 has been fabricated through a simple solvothermal method by modifying the molar ratio of the initial raw materials. The products are characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). It is found that the two phases, Ni2P and Ni12P5, are interlaced with one another in the as-formed nanocomposite, resulting in more interfaces. The bi-phase catalyst exhibits a markedly enhanced catalytic activity in the reduction of 4-nitrophenol, as compared to that of single Ni2P or Ni12P5. The enhanced catalytic activity can be attributed to the unique n–n series effects, which result in the increased ease of electron transfer over the Ni2P/Ni12P5 bi-phase catalyst.


Introduction

Over the years, environmental pollution is increasing due to the fast growth of diverse industries and the release of various types of contaminants into water. Among these toxic effluents, 4-nitrophenol (4-NP), which is generally used in the production of pesticides, dyes and so on, has been proven to be very harmful to plants, animals and humans.1–3 However, there is a broad consensus on the difficult detoxification of 4-NP by conventional water treatment due to its high chemical stability and resistance to microbial degradation which are attributed to the presence of a nitro-group in the aromatic compound.4 So the effective removal of 4-NP is imminent and has received a lot of attention. 4-Aminophenol (4-AP) is an important industrial precursor material in pharmaceutical, photographic and plastic fields.2,4–7 Traditionally, 4-AP could be produced by the reduction of 4-NP in the presence of a reducing agent.3,8 Therefore, the catalytic reduction of 4-NP to 4-AP can not only prevent its toxic effects on the ambient conditions, but also meet the great demand for 4-AP.

The early conventional methods for the reduction of 4-NP to 4-AP using an iron/acid as a reducing agent cause a serious pollution problem. For this reason, a series of noble metal-based catalysts in the presence of NaBH4, such as gold-, silver-, palladium-, and aurum-based systems,3,5–9 have been well investigated for the reduction of 4-NP over the past few decades due to their good performance in catalysis. However, some unavoidable problems of noble metal systems exist, including high cost, interparticle aggregation of metal nanoparticles, no reuse and other defects, which limit their large-scale commercialization.2,10–12 Therefore, it is still necessary to develop more low-cost, stable, reusable and noble metal free catalysts. In this context, the direct reduction of 4-NP catalyzed by semiconductor materials is considered as an alternative green process.13 Naturally, a lot of effort has been made to explore new semiconductor catalysts for the reduction of 4-NP to 4-AP.14–17 Nickel phosphides are earth-abundant, inexpensive, electrical conductors and usually possess metallic character and high thermal and chemical stabilities, which thus attract much attention as catalytic materials.18,19 Some nickel phosphide catalysts with different stoichiometries, such as Ni2P,20,21 Ni5P4,22 and Ni12P5,23 have been reported and studied. In recent years, nickel phosphides have been proverbially investigated for hydride sulfurization,24,25 hydrogen evolution reaction,26 hydrogenation,27 and oxygen evolution reaction,28 revealing high activity and excellent stability. Meanwhile, Du and co-workers reported a series of metal phosphide catalysts,29–33 among which nickel phosphide (Ni2P) was proved to be an efficient co-catalyst, resulting in an extension application in photocatalytic activities for water splitting. Recently, nickel phosphide (Ni2P), coupled with CdS, was also used in the photo-driven reduction of nitroaranes.34 The high catalytic activity of metal phosphides could be ascribed to the introduction of phosphorus (P), which greatly influences the active metal sites and the metal–P bonds.22,32 It is significant that nickel phosphides with these excellent functions deserve further exploration and application in other fields.

With this in mind, we report a one-pot facile solvothermal method to synthesize nanostructured nickel phosphide materials for the reduction of 4-NP. By modifying the molar ratio of the initial raw materials, Ni2P, Ni12P5 and Ni2P/Ni12P5 bi-phase nanocomposites were successfully synthesized. We further investigated the catalytic performance of the three nanostructured nickel phosphide materials in the catalytic reduction of 4-NP in the presence of KBH4. Interestingly, enhanced catalytic efficiency was observed for the bi-phase nanocomposite compared with mono-phase nickel phosphide. Furthermore, the kinetics and mechanism of the catalytic reduction of 4-NP over the nickel phosphide catalyst are discussed. To the best of our knowledge, there are yet no reports on the nickel phosphide nanocomposite as a heterogeneous catalyst for the reduction of 4-NP without light irradiation.

Experimental section

Synthesis

All chemicals were used as received without further purification. Deionized (DI) water was used throughout. In a typical procedure, a certain amount of NiCl2·6H2O was dissolved in a solution containing 20 mL DI water and 10 mL ethanol. Then a calculated amount of ground red phosphorus was added to the solution. The mixed solution was stirred at room temperature for 30 minutes. The resulting mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and sealed tightly. The autoclave was then heated to 180 °C and kept there for 16 h. The black precipitate was filtered and washed three times with DI water and ethanol. Afterwards, the as-prepared samples were dried at 60 °C. The initial molar ratio of phosphorus and NiCl2·6H2O was controlled, including 1[thin space (1/6-em)]:[thin space (1/6-em)]1, 2[thin space (1/6-em)]:[thin space (1/6-em)]1 and 4[thin space (1/6-em)]:[thin space (1/6-em)]1, and the as-prepared samples are marked as Ni12P5, Ni2P/Ni12P5 and Ni2P, respectively.

Characterization

X-ray diffraction (XRD) was employed to characterize the crystal structures of the samples. The morphologies of the as-synthesized samples were characterized by field emission scanning electron microscopy (FESEM, JSM-7500F). The transmission electron microscopy (TEM) measurements were conducted using a JEM-2010. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a PHI 5000 instrument with an Al KR anode. Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halender (BJH) methods were applied to calculate the specific surface area and pore size distribution, respectively. Absorbance spectra were recorded using a SHIMADZU UV-2550 UV-vis spectrophotometer. Electrochemical impedance spectroscopy (EIS) measurements were conducted on a CHI 660D electrochemical work station with a conventional three-electrode configuration. A Pt electrode and a saturated calomel electrode were used as the counter electrode and the reference electrode, and a Na2SO4 (0.1 M) aqueous solution was used as the electrolyte. The Mott–Schottky (MS) method was employed at a frequency of 1.0 kHz.

Catalytic reduction of 4-NP

A 14 mg L−1 4-NP solution was prepared by dissolving 14 mg 4-NP into 1000 mL water at room temperature. Then, 1.5 mg KBH4 was added to a quartz cuvette containing 3 mL 4-NP solution. The UV spectra of the mixture were recorded every minute in the range of 250–500 nm after 1.5 mg catalyst was added to the above solution. The catalytic reduction of o-nitrophenol (o-NP) and 3-nitrophenol (3-NP) was also conducted under the same conditions in addition to using different concentrations of o-NP (100 mg L−1) and 3-NP (100 mg L−1). The catalyst was reused as follows. The particles were collected after one course of reaction and washed thoroughly with deionized water. After being dried at room temperature, the catalyst was subjected to subsequent cycles to study its stability and reusability.

Results and discussion

Fig. 1a shows the XRD patterns of the three catalysts with different P/Ni molar ratios. For Ni12P5 (P/Ni = 1[thin space (1/6-em)]:[thin space (1/6-em)]1), the XRD pattern exhibited only Ni12P5 diffraction peaks at 32.7°, 35.8°, 38.4°, 41.8°, 44.4°, 47.0°, 49.0°, 54.0° and 56.2° (JCPDS no. 65-1623). Peaks at 40.8°, 44.6°, 47.3°, 54.4°, 66.4°, 72.7°, and 75.0° in the XRD pattern of Ni2P (P/Ni = 4[thin space (1/6-em)]:[thin space (1/6-em)]1) could be attributed to Ni2P (JCPDS no. 65-3544). The diffraction peaks from both Ni12P5 and Ni2P were observed in the patterns of Ni2P/Ni12P5 (P/Ni = 2[thin space (1/6-em)]:[thin space (1/6-em)]1), indicating that this sample expressed a mixed phase structure. It is supposed that the Ni12P5 fractions were more than those of Ni2P in the Ni2P/Ni12P5 sample as reflected by the peak intensity ratio. And the XRD patterns ranging from 38° to 52° are presented in great detail in Fig. 1b.
image file: c7dt02375h-f1.tif
Fig. 1 (a) XRD patterns of the three catalysts (Ni12P5, Ni2P/Ni12P5, and Ni2P) with different P/Ni molar ratios, and (b) the details of a part of the XRD patterns.

The structure and morphology of the three as-obtained Ni12P5, Ni2P/Ni12P5, and Ni2P samples were investigated by FE-SEM and TEM. Fig. 2a and b show the low- and high-magnification FE-SEM images of the Ni2P/Ni12P5 sample, respectively. It can be clearly seen that the sample was composed of agglomerated nanoparticles, and presented special dumpling-like structures. The SEM images of the Ni12P5 and Ni2P samples are also shown in Fig. S1, and the agglomeration of nanoparticles also occurred. More structural details can be seen in the TEM image (Fig. 2c). The TEM image revealed that the “dumpling” was composed of numerous primary crystallites of 10–20 nm size, where a few pores were left between some adjacent crystallites. The EDX result of the sample which is shown in Fig. S2 conveys the chemical composition of the Ni2P/Ni12P5 sample, and only Ni and P were detected. HRTEM was used in order to obtain further structure information; the HRTEM image (Fig. 2d) displays mixed crystalline Ni2P phase and Ni12P5 phase. The fringes in a typical HRTEM image (Fig. 2d) are separated by 0.241 nm and 0.202 nm, which are assigned to the (112) and (021) lattice spacings of the Ni12P5 phase, respectively. Another interplanar spacing of 0.330 nm corresponded to the Ni2P (001) space. Interestingly, the two phases are interlaced with each other and are similar to piecing together like a jigsaw puzzle. The above results also suggest that the Ni2P/Ni12P5 sample obtained is made up by two different phases, which coincides well with the former XRD patterns.


image file: c7dt02375h-f2.tif
Fig. 2 (a, b) FE-SEM images, (c) TEM image, and (d) HRTEM image of the Ni2P/Ni12P5 sample.

It is generally believed that the catalytic performance of nanomaterials is related to their BET surface areas and component parts. Fig. S3 displays the N2 adsorption–desorption isotherms and the corresponding pore size distribution curves (inset) for the three samples. As calculated by the Brunauer–Emmett–Teller (BET) method, the BET surface area of Ni12P5 is 0.95 m2 g−1, which is much smaller than those of Ni2P/Ni12P5 (5.66 m2 g−1) and Ni2P (5.70 m2 g−1). To a certain extent, there is no doubt that the poor catalytic activity of Ni12P5 could be attributed to its low BET surface area, while Ni2P/Ni12P5, with the same BET surface area and pore size distribution as Ni2P, exhibited a prominent catalytic activity. These results suggest that the better performance of the Ni2P/Ni12P5 sample is mainly attributed to the introduction of the Ni12P5 phase.

The surface electronic state and the chemical composition of the as-prepared samples were explored by XPS. The XPS spectra in the Ni (2p) and P (2p) regions for the three catalysts are shown in Fig. 3a and b, respectively. For the Ni2P catalyst, the peaks at 856.0 eV and 133.6 eV were assigned to the Ni2+ and P5+ species,35 respectively. A binding energy of 853.9 eV implies that the peak located is close to that of Ni0 (852.8 eV), suggesting that the Ni in phosphide has a small partial positive charge (Niδ+, 0 < δ < 2).24 Similarly, another peak of P 2p at 129.7 eV can be assigned to P in phosphide, which suggests that the related P species have a partial negative charge (Pδ, 0 < δ < 1), because this binding energy is very close to that of P0 (130.0 eV).24 For the Ni12P5 sample, the peak at 852.9 eV was attributed to Niδ+ in Ni12P5, and the P (2p3/2) peak at 129.9 eV was assigned to Pδ in Ni12P5. The XPS spectra of Ni2P/Ni12P5 were different from those of the other two catalysts, showing two Ni (2p3/2) peaks at 853.9 eV and 852.9 eV, which could be assigned to Niδ+ in Ni2P and Ni12P5, respectively. For all of the catalysts, the Ni (2p3/2) binding energies characteristic of Ni2+ are basically the same, located at 856.0 eV, while the P (2p3/2) binding energies characteristic of P5+ are significantly different, being attributed to the formation of low valence P.36


image file: c7dt02375h-f3.tif
Fig. 3 (a) High resolution XPS spectra of Ni 2p, and (b) P 2p.

Catalytic activity

The catalytic activities of the as-prepared catalysts were tested through the reduction of 4-NP nitro-aromatic in the presence of KBH4 as the reductant. The initial light yellow 4-NP solution exhibited a strong absorption peak at 317 nm, which visibly shifted to 400 nm (simply shown in Fig. 4a) once KBH4 participated, and the color changed to yellow–green, indicating the formation of 4-nitrophenolate ions.37,38 Only KBH4 does not stimulate a visible reaction of 4-NP, which could be negligible in comparison with that in the absence of the catalyst. The catalytic reduction could be observed by the decline of the peak at 400 nm after the addition of the catalyst. Also, the increase of another peak at 295 nm also occurred, which can be attributed to the formation of 4-AP.2,3,15Fig. 4a–c present the absorbance spectra versus wavelength plots at various times for the reduction reaction of 4-NP to 4-AP in the presence of Ni12P5, Ni2P/Ni12P5, and Ni2P catalysts, respectively. Fig. 4d reflects the variation in 4-NP concentration (C/C0) versus reaction time during the 4-NP reduction process. It is worth noting that the enhancement in the catalytic activity of Ni2P/Ni12P5 is evident and the efficiency of the reduction of 4-NP reaches its maximum value. The catalytic activities of the three catalysts followed the sequence Ni2P/Ni12P5 > Ni2P > Ni12P5, which demonstrates that the Ni2P/Ni12P5 bi-phase heterostructure was the optimal choice for this system.
image file: c7dt02375h-f4.tif
Fig. 4 UV–vis absorption spectra during the catalytic reduction of 4-NP, (a) Ni12P5, (b) Ni2P/Ni12P5, and (c) Ni2P, and (d) C/C0versus the reaction time for the reduction of 4-NP with different catalysts.

To further investigate the reduction rate of 4-NP, the pseudo-first-order kinetics can be applied in this system, as the concentration of KBH4 greatly exceeds that of 4-NP and the reduction rate can be assumed to have no relationship with the concentration of KBH4:

image file: c7dt02375h-t1.tif
where C0 and C are the concentrations of 4-NP at the beginning and at time t, respectively. The relationship of ln(C/C0) versus time was calculated and is shown in Fig. 5a. The observed rate constants for Ni12P5, Ni2P/Ni12P5, and Ni2P were 4.17 × 10−3 s−1, 8.34 × 10−3 s−1 and 5.0 × 10−3 s−1, respectively. These results clearly indicate that the Ni2P/Ni12P5 bi-phase hetero-structure is the superior catalyst which can enhance the catalytic efficiency. It is worth mentioning that the catalytic activity of Ni2P/Ni12P5 is even higher than those of some reported catalysts including noble metal-based and metal-free catalysts (Table 1).


image file: c7dt02375h-f5.tif
Fig. 5 (a) ln(C/C0) versus time during the course of the reduction of 4-NP with different catalysts, and (b) a catalytic stability test of the Ni2P/Ni12P5 sample.
Table 1 Comparison of k of different catalysts for the catalytic reduction of 4-NP
Sample k × 10−3 (s−1) Ref.
CNFs/AgNPs 6.2 2
Pd0.10/G 2.4 5
Au-Fe3O4 6.3 9
Ag/GO 8.2 10
Au/g-C3N4-6 8.0 16
Au(10)/TiO2 4.1 17
RGO-Ni25Co75 1.6 39
Pd-graphene nanohybrid 2.35 40
Ag-nanoparticle/C 1.69 41
Pd-rGO 4.5 42
Ni2P/Ni12P5 8.34 This work


Furthermore, we investigated the stability of the Ni2P/Ni12P5 sample, and the recycling experiment was conducted. Fig. 5b shows the cycling runs of the Ni2P/Ni12P5 catalyst. It could be seen that the catalyst retained high catalytic activity after five cycles in the reduction reaction of 4-NP, and this good performance stability would eventually take it a step closer to industrial applications. These results demonstrate that the use of the Ni2P/Ni12P5 bi-phase hetero-structure catalyst could be a promising approach for the reduction of 4-NP in waste water due to its high catalytic efficiency and good stability.

Similar reduction patterns for o-NP and 3-NP are also observed. The rate constants for o-NP and 3-NP reduction catalyzed by the three catalysts are shown in Fig. S4. The same conclusion that the Ni2P/Ni12P5 bi-phase hetero-structure is the most prominent one can be drawn from the experimental results. It was reasonably proposed that Ni2P and Ni12P5 had a synergetic effect, which accounted for the high catalytic activity of Ni2P/Ni12P5 catalysts.

Mechanism considerations

The flat-band positions of Ni12P5 and Ni2P can be estimated from the linear potential plot based on the Mott–Schotty equation. The positive slopes of the linear lines at a frequency of 1.0 kHz are shown in Fig. 6, indicating that Ni2P and Ni12P5 are both n-type semiconductors.43,44 According to the flat-band potential that was measured, the lowest CB potentials of Ni2P and Ni12P5 are calculated to be approximately 0.19 V and 0.26 V (vs. SCE), respectively. This n–n semiconductor heterojunction composed of two different phases, Ni2P and Ni12P5, with different electron affinities, can greatly facilitate electron transfer, thus enhancing the catalytic reduction of 4-NP.
image file: c7dt02375h-f6.tif
Fig. 6 Mott–Schottky plots of Ni12P5 and Ni2P for determining the flat-band potentials of the samples.

To understand the heterojunction effect on the remarkable catalytic activity of the Ni2P/Ni12P5 bi-phase nanohybrid, the electrochemical impedance spectra of all the products were measured. Fig. 7 shows representative EIS Nyquist plots of the Ni12P5, Ni2P/Ni12P5 and Ni2P samples. It is clear that the arc radius of Ni2P/Ni12P5 is much smaller than those of Ni12P5 and Ni2P. Generally, a smaller semicircle in the EIS Nyquist plot denotes faster interfacial electron transfer.20 The difference just reflects a remarkable decrease of the charge transfer resistance of Ni2P/Ni12P5 which is due to the series effect between the Ni2P phase and the Ni12P5 phase. The conclusion drawn from the EIS Nyquist plot has also proved that the formed n–n heterojunction is conducive to electronic transfer.


image file: c7dt02375h-f7.tif
Fig. 7 EIS Nyquist plots of Ni12P5, Ni2P/Ni12P5 and Ni2P samples.

The catalyst is considered as the electron transfer media between the BH4 donor and the acceptor molecules of 4-NP, and the electron transfer rate plays a decisive role during the 4-NP catalytic reduction process.1 In this work, the Ni2P/Ni12P5 bi-phase sample exhibited better catalytic performance than those of Ni2P and Ni12P5 alone. The postulate mechanism of the catalytic reduction of 4-NP over Ni2P/Ni12P5 with KBH4 is simply presented in Fig. 8 according to the above results and previous studies.2,45 The dispersed Ni2P/Ni12P5 bi-phase catalyst in a solution containing KBH4 and 4-NP is negatively charged, with a zeta potential of about −19.30 mV. Accordingly, the effective adsorption of 4-NP molecules, being slightly acidic, can be greatly improved,15 leading to a better catalytic performance. Furthermore, based on the above results, it is evident that the enhanced activity of the hybrid catalyst involving Ni2P and Ni12P5 can be attributed to the special series synergetic effect caused by the n–n heterojunction, which has a certain characteristic of lower charge transfer resistance and can accelerate the electron transfer between BH4 and 4-NP. The ease of electron transfer over the Ni2P/Ni12P5 bi-phase catalyst makes it an ideal mediator for the further reduction process and results in the efficient acceptance of electrons by 4-NP molecules and its conversion to 4-AP. The noticeable influencing factors of Ni2P/Ni12P5 bi-phase heterojunction in situ generation could develop a large number of interfaces. The greater the number of interfaces, the more such series synergetic effects. Hence, the excellent catalytic performance of the Ni2P/Ni12P5 bi-phase heterojunction with KBH4 is ascribed to the effective adsorption at the surface and the unique n–n series effects of Ni2P and Ni12P5. To gain further insights into the detailed mechanism, more work will be conducted.


image file: c7dt02375h-f8.tif
Fig. 8 Postulate mechanism of the 4-NP catalytic reduction with the Ni2P/Ni12P5 bi-phase composite.

Conclusion

A Ni2P/Ni12P5 bi-phase nanocomposite was fabricated by a solvothermal method in situ, resulting in an intimate contact between the Ni2P phase and the Ni12P5 phase. Catalytic reduction reactions of nitrophenols were carried out in the presence of KBH4, and the Ni2P/Ni12P5 bi-phase nanohybrid exhibited much higher activity compared with the bare Ni2P and Ni12P5 and excellent stability. The results shown in this study clearly demonstrated that the formation of unique n–n heterojunctions between Ni2P and Ni12P5 is beneficial to the electron transfer. Consequently, the high catalytic performance of the Ni2P/Ni12P5 bi-phase nanocomposite in the nitrophenol reduction process is attributed to the synergistic chemical adsorption and particular n–n series effects. The as-prepared Ni2P/Ni12P5 bi-phase catalyst is stable, efficient and eco-friendly, and thus has great potential to be an alternative high-performance catalyst for practical applications. This work may lead to more detail research of nickel phosphide and also broaden its application in many fields.

Acknowledgements

This work was financially supported by the NSF of China (Nos. 51572152, 31373122, and 51502155) and the NSRF of Hubei Provincial Education Office of China (No. D20151203).

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Footnote

Electronic supplementary information (ESI) available: FE-SEM images, N2 adsorption–desorption isotherms and UV-vis spectra. See DOI: 10.1039/c7dt02375h

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