Intercalation Synthesis of Prussian Blue Analogue Nanocone and

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Intercalation Synthesis of Prussian Blue Analog Nanocone and Their Conversion into Fe Doped CoxP Nanocone for Enhanced Hydrogen Evolution Xiaosong Guo, Xiaoguang Yu, Zijia Feng, Jun Liang, Qinglin Li, Zezhong Lv, Bingjie Liu, Chuncheng Hao, and Guicun Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Sustainable Chemistry & Engineering

Intercalation Synthesis of Prussian Blue Analog Nanocone and Their Conversion into Fe Doped CoxP Nanocone for Enhanced Hydrogen Evolution Xiaosong Guo†, Xiaoguang Yu†, Zijia Feng†, Jun Liang†, Qinglin Li†, Zezhong Lv†, Bingjie Liu†, Chuncheng Hao*,†,‡, Guicun Li*,† †

College of Materials Science and Engineering, Qingdao University of Science and

Technology, No.53 Zhengzhou Road, Qingdao, Shandong, 266042, People's Republic of China ‡

State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong

University, No.28, Xianning West Road, Xi'an, Shaanxi, 710049, People's Republic of China

*E-mail: [email protected]; [email protected].

Fax: 86-532-84022814. Tel: 86-532-84022632

Abstract Compared with the monometallic phosphides, bimetallic phosphides can further improve the catalytic performance for hydrogen evolution reaction (HER). As such, the rational design and facile synthesis of bimetallic-based phosphides with well controlled architectures and compositions is of scientific and technological importance. In this work, Fe-Co prussian blue analogue (PBA) nanocones (NCs) have been successfully fabricated via an intercalation reaction strategy by utilizing layer

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structured α-Co(OH)2 NCs as self-sacrificing templates. After calcination and phosphorization process, Fe-Co PBA NCs can be converted to Fe doped CoxP NCs without obvious shrinkage. Electrochemical tests show that Fe incorporation can effectively promote the electrocatalytic activities of CoxP. This simple and effective method will be benefit for the development of other functional Co based bimetallic compounds. Furthermore, this strategy can possibly be extended to fabricate a series of PBA material with special structure and novel morphology, which can be served as a promising platform for diverse applications, especially in energy storage and conversion.

Keywords prussian blue analogue, nanocones, anion exchange reaction, intercalation reaction, hydrogen evolution reaction.

Introduction Due to its highly effective and environmentally friendly nature, hydrogen is utilized as a promising energy carrier and important chemical building block to substitute for fossil fuels in the future.[1–2] In recent years, electrochemical splitting of water provides a flexible and sustainable way to produce clean hydrogen energy.[3–4] Nowadays, Pt-based materials are the most active electrocatalysts for HER. However, the scarcity and high cost limit their practical application in energy field.[5–7] Therefore, designing and developing non-noble metal HER electrocatalysts are being actively conducted for the future of hydrogen economy, and have attracted tremendous research interests. Currently, several kinds of transition metal compounds,


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such as phosphides, selenides, sulphides, borides, carbides, and metal alloys, have been identified as promising candidates for HER.[8–21] In these materials, the transition metal phosphides including iron, cobalt, nickel, molybdenum and tungsten phosphides have been studied in recent years because of their environmental friendliness and excellent electrocatalytic performance. [22–34] Optimizing the electronic structure of electrocatalyst plays a pivotal role in obtaining better HER performance. The electronic structure can be regulated by coordination of multiple elements.[35–38] Compared with the monometallic phosphides, binary or ternary transition metal phosphides can further improve the catalytic performance for HER due to the synergistic effect of different components.[39–41] Inspired by this, various strategies have been exploited to construct solid-solution structure and realize heteroatom doping.[42–44] For example, Jaramillo and co-workers demonstrated that Fe substitution of Co in CoP can optimize hydrogen adsorption free energy on the catalyst surface.[45] Sun and co-workers synthesized Fe-Co bimetallic phosphides with nanowire array structure on Ti foil and achieved excellent catalytic activities.[46] Zhang and co-workers utilized a nanocasting method to fabricate CoP/FeP/graphene as a highly active and durable electrocatalyst for the HER.[47] Although these intriguing achievements have shown that the incorporation of Fe can enhance the activity of Co-based phosphides electrocatalysts for HER, there are few reports about the rational design and facile synthesis of Fe-Co bimetallic phosphides with well controlled architectures and compositions. Dodecyl sulfate (DS−) intercalated α-Co(OH)2 nanocones (DS−-Co(OH)2 NCs) is a



class of lamellar compounds, which consists of positively charged brucite-like Co(OH)2 host layers and DS− anions located in the interlayer gallery for charge balance. These intercalated DS− anions can exchange for various anions, thus making it possible to realize heteroatom doping through anion exchange reaction with minimal change to the structure.[48] Based on above analysis, [Fe(CN)6]3− and MoO42– intercalated α-Co(OH)2 NCs are fabricated through exchanging intercalated DS− anions for [Fe(CN)6]3− and MoO42– anions, respectively. Another amazing thing is that the intercalated [Fe(CN)6]3− anions can further in situ react with Co(OH)2 host layers to form more stable Fe-Co PBA nanoparticles (NPs) while the intercalated MoO42– anions cannot. The layered structures of α-Co(OH)2 can efficiently prevent the aggregation and excessive growth of Fe-Co PBA NPs, and finally Fe-Co PBA NCs are obtained with well-preserved morphology. After calcination and phosphorization process, Fe-Co PBA NCs can be converted to Fe doped CoP NCs without any obvious shrinkage. Notably, this intercalation reaction strategy developed here can realize Fe incorporation at the atomic level. Electrochemical tests show that Fe incorporation can effectively enhance the HER activity of CoP. This is the original work of synthesizing Fe doped CoP NCs through an efficient anion exchange reaction, which will be benefit for the development of other functional transition metal complex for various applications. Besides, this mechanism discussed in this paper can be applicable to fabricate a series of PBA material with special structure and novel morphology, which may open an avenue in the design of PBA material.

Results and discussion


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DS−-Co(OH)2 NCs are successfully prepared via a solution-based reaction of cobalt chloride and urea in the presence of sodium dodecyl sulfate (see the Supporting Information for details). The scanning electron microscopy (SEM) image of the product is shown in Figure 1a, it can be seen that α-Co(OH)2 with conical morphology is synthesized. The high-magnification SEM image shown in Figure 1b reveals the wrinkled surface of α-Co(OH)2 NCs, which suggests the layered structure. The transmission electron microscopy (TEM) image in Figure 1c further confirms the hollow nature of obtained α-Co(OH)2 NCs. The corresponding TEM image (Figure 1d) and high-resolution TEM (HRTEM) image (Figure S1) display serrated edge of the cone wall, indicating the staggered stacking of nanosheets with an interlayer spacing of 2.2 nm. The selected area electron diffraction (SAED) pattern of α-Co(OH)2 NCs is shown in Figure 1e, which can be indexed to in-plane diffraction rings of turbostratic Co(OH)2 host layers.[49] The layered structure of α-Co(OH)2 NCs is also identified by X-ray diffraction (XRD) in Figure 1f. All the diffraction peaks in the low-angle region (2θ < 25°) indicate the regular arrangement of (00l) planes and the layered structure of NCs, while the asymmetric nature of (100) and (110) peaks indicate the turbostratic structure of Co(OH)2 host layers.[50–52]



Figure 1. (a), (b) SEM images of DS−-Co(OH)2 NCs; TEM images of (c) DS−-Co(OH)2 NCs and (d) serrated edge of DS−-Co(OH)2 NCs; (e) SAED and (f) XRD patterns of DS−-Co(OH)2 NCs. Figure 2a exhibits the Fourier transform infrared (FT-IR) spectra of DS−-Co(OH)2 NCs before and after reacting with K3[Fe(CN)6]. In all spectra, a broad band around 3444 cm−1 is attributed to the stretching vibration of O–H bonds, which indicates the presence of hydroxyl groups and interlayer water molecules. The weak band around 1625 cm−1 is the bending mode of the water molecules.[53] The absorption bands at low-frequency regions (below 650 cm−1) are ascribed to Co–O stretching and Co–OH bending vibrations in the host layers.[54–55] Besides these common bands, the interlayer DS− anions are confirmed by the appearance of bands at 2921 and 2852 cm−1, which are characteristic of the asymmetric and symmetric CH2 stretching vibrations in the alkyl chain of DS− anions, respectively, while the weak band at 2959 cm−1 is assigned to the stretching vibration of CH3 group of DS− anions. The bands in the range of 1300–900 cm−1 are associated with the stretching modes of sulfate group in DS− anions.[52] After the anion exchange reaction, these characteristic bands of DS−


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anions weaken or disappear, whereas two additional sharp bands at 594 and 2085 cm−1 are observed, which are attributed to the vibration of Fe–CN bond and C≡N bond, respectively.[56] The crystallographic structure of Fe(CN)6]3− intercalated α-Co(OH)2 NCs is examined by powder XRD, which is shown in Figure 2b. All diffraction peaks can be unambiguously assigned to K2CoFe(CN)6 (JCPDS card no. 75-0038). The characteristic peaks corresponding to layered structure disappear, confirming layered structure of NCs is destroyed during the anion exchange interaction process. Moreover, the disappearance of in-plane diffraction peaks of (100) and (110) indicates that the Co(OH)2 host layers are basically destroyed. There results suggest [Fe(CN)6]3− anions not only exchange with DS− anions but also further react with in-plane Co(OH)2 layers to form K2CoFe(CN)6. Inductively coupled plasma mass spectrometry (ICP-MS) is utilized to determine the exact composition of obtained NCs, which exhibits the atomic ratio of Co to Fe element is 5.5:1. Thus, it can be speculated that only a portion of Co(OH)2 reacts with [Fe(CN)6]3− anions to form K2CoFe(CN)6, and ultimately K2CoFe(CN)6/Co(OH)2 hybrid NCs is obtained. The color change of obtained sample from initial green to black also proves the formation of K2CoFe(CN)6, which is shown in Figure S2. Figure S3 shows the SEM image of Fe-Co PBA NCs, it can be observed that the conical morphology is well preserved. In contrast with the smooth and homogeneous morphology of Co(OH)2 NCs, Fe-Co PBA NCs with a rough surface is decorated randomly by nanoparticles (Figures 2c and 2d), suggesting a gradual phase conversion from Co(OH)2 to PBA occurs after the anion exchange reaction process. The comparasion of EDS images of



Co(OH)2 and Fe-Co PBA NCs confirms the successful incorporation and homogeneous distribution of Fe element throughout the whole NC (Figure 2e). TEM image (Figure 2f) exhibits that Fe-Co PBA NCs are consist of nanoparticles and the hollow interior is perfectly preserved. The dark spots with an average size of 2 nm in TEM images (Figure 2g) is presumably K2CoFe(CN)6 nanoparticles, which is surrounded by lighter regions of Co(OH)2. These K2CoFe(CN)6 nanoparticles uniformly disperse over the whole NC without any obvious aggregation, resulting in the destruction of in-plane structure of Co(OH)2 host layers. The HRTEM image in Figure 2h shows the lattice parameter of the (420) plane and (400) plane of K2CoFe(CN)6 are 0.225 and 0.252 nm, respectively. Faint and diffraction rings can be observed in SAED pattern (Figure S4), disclosing polycrystalline character of the Fe-Co PBA NCs.


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Figure 2. (a) FT-IR spectra of Co(OH)2 and Fe-Co PBA NCs; (b) XRD pattern of Fe-Co PBA NCs; (c), (d) SEM images of Fe-Co PBA NCs; (e) EDS images of Co(OH)2 and Fe-Co PBA NCs; (f), (g) TEM and (h) HRTEM images of Fe-Co PBA NCs. The thermal decomposition behaviors of Co(OH)2 and Fe-Co PBA NCs are investigated with thermogravimetric analysis (TGA) in air, as shown in Figure S5. In the initial stage of both curves, the gradual mass loss below 150 °C is caused by the evaporation of physically adsorbed water and intercalated water molecules. Thereafter, in the following stage between 150 and 235 °C, the loss generated from dehydroxylation of Co(OH)2 indicates that the obtained Fe-Co PBA NCs includes unreacted Co(OH)2, which is consistent with the above analysis. After that, the decomposition behavior becomes different. Co(OH)2 NCs exhibit a major decrease between 235 and 700 °C, which can be mainly attributed to the combustion of DS− anions.

Figure 3. (a) XRD patterns of Co3O4 and Fe-Co3O4 NCs; SEM images of (b), (c) Fe-Co3O4 NCs and (d), (e) Co3O4 NCs. After calcining at 450 °C in air, Co(OH)2 and Fe-Co PBA NCs are totally transformed



into Co3O4 and Fe-Co3O4 NCs, respectively, being confirmed by XRD measurement. Figure 3a displays XRD patterns of Co3O4 and Fe-Co3O4 NCs, all of the reflections can be indexed as a face-centered cubic phase of spinel Co3O4 (JCPDS card no. 43–1003), indicating that the spinel structure is retained after Fe doped. Besides, a slight peak shift to lower 2theta is observed in the XRD pattern of Fe-Co3O4 without any peaks regarding Fe oxides phases, indicating highly controlled incorporation of Fe element into the Co3O4 lattice.[57–58] Figure 3b and 3c depict typical SEM images of Fe-Co3O4 NCs, it can be seen that the original conical framework with hollow interior is retained well without any shrink. In contrast, the obvious shrinkage of Co3O4 NCs toward the center is manifested in Figure 3d and 3e, which is attributed to the pyrolysis of intercalated DS− anions and Co(OH)2 host layer.

Figure 4. (a) XRD pattern of Mo-Co NCs; (b) TEM image of side edge of Mo-Co NCs; (c) TEM and (d) SEM images of Mo-Co NCs; (e) XRD pattern and (f) SEM image of Mo-Co3O4 NCs. In order to investigate this transformation mechanism, MoO42– intercalated


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α-Co(OH)2 NCs (Mo-Co NCs) are synthesized by exchanging with MoO42– anions to further study this intercalation reaction. The change of the functional group is evidenced by FT-IR (Figure S6), it can be found that the features of DS− anions disappeared and a series of absorption bands in the 500–910 cm−1 range corresponding to Mo-O complexes become obvious.[59] The color of obtained sample turning into dark green suggests the change of intercalated anions, which is shown in Figure S2c. The incorporation of Mo in the NCs is further confirmed by EDS analysis (Figure S7). These evidences confirm successful intercalation of MoO42− groups through anion exchange process. XRD pattern of Mo-Co NCs in Figure 4a exhibits the basal diffraction peaks corresponding to regular arrangement of (00l) planes almost disappear and only a weak peak for (003) at 9.5° can be observed, deducing that exchanging with MoO42– anions may cause damage to order degree of layered structure of Co(OH)2 NCs due to the different size of intercalated anions. The TEM image of side edge of Mo-Co NCs (Figure 4b) further verifies an obvious decrease in the order degree of layered structure. In addition, it is notable that in-plane diffraction peaks of (100) and (110) retained their original positions, indicating that the Co(OH)2 host layers are basically intact, consistent with the SAED pattern (Figure S8). The layered structure of NC bottom edge shown in TEM image (Figure S9) also confirms no further reaction occurs between Co(OH)2 host layers and MoO42- anions after anion exchange process, which is different from the granular microstructure of Fe-Co PBA NCs. The TEM (Figure 4c) and SEM images (Figure 4d) reveal that the conical structure is preserved well after the anion exchange process. These above results



suggest that no further reaction occurrs after MoO42– anions exchanges with DS− anions. After calcined in air, the Co(OH)2 host layers are transformed to face-centered cubic phase of spinel Co3O4 while a portion of Co(OH)2 reacts with MoO42– to form CoMoO4, which are certified by XRD result in Figure 4e. It is more interesting to note that the Mo-Co NCs shows shrinkage during calcination (Figure 4f), which manifest such a shrinkage phenomenon is mainly resulted from the converting of Co(OH)2 host layers into Co3O4. Unlike MoO42– anions, the [Fe(CN)6]3− anions can not only exchange with DS− anions, but also in situ react with Co(OH)2 host layers to form K2CoFe(CN)6. To further clarify the role of anion exchange in this process, β-Co(OH)2 hexagonal platelet without intercalated DS− anions is synthesized to react with [Fe(CN)6]3− anions, which is denoted as Fe-β-Co(OH)2. SEM and TEM images (Figure S10) reveal some large square PBA particles are deposited on the β-Co(OH)2 platelet. In contrast with the diffraction peaks of Fe-Co PBA NCs and β-Co(OH)2, a superposed XRD pattern of β-Co(OH)2 and K2CoFe(CN)6 is observed for Fe-β-Co(OH)2 (Figure S11), illustrating an in situ growth of K2CoFe(CN)6 on the surface of β-Co(OH)2 hexagonal platelet. Therefore, it can be concluded that ione exchange plays an important role in transforming from DS−-Co(OH)2 NCs to K2CoFe(CN)6.


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Figure 5. Schematic showing the mechanism for the transformation of Co(OH)2 NCs into Fe-Co PBA NCs. On the basis of above analysis, the mechanism for the transformation of Co(OH)2 NCs into hierarchical Fe-Co PBA NCs involves an anion exchange reaction and in situ reaction between Co(OH)2 host layers and intercalated [Fe(CN)6]3− anions. A schematic illustration is shown to explain the formation of Fe-Co PBA NCs (Figure 5). First, the DS−-Co(OH)2 NCs are transformed to [Fe(CN)6]3− intercalated α-Co(OH)2 NCs by anions exchange DS− anions with [Fe(CN)6]3− anions. Then, intercalated [Fe(CN)6]3− anions react with adjacent Co(OH)2 host layers to form more stable PBA phase, which converts the microsheets into nanoparticles (NPs). In comparison to conventional co-precipitation method for synthesizing PBA materials, no Co2+ ion participates in this solid-liquid reaction, restricting further growth of PBA NPs. In this process, N ends of CN groups in [Fe(CN)6]3− anions may coordinate to cobalt atoms with tetrahedral coordination, which realizes the high uniform of Fe doped at the atomic level.



Figure 6. (a) SEM image of Fe-CoxP NCs; (b) XRD patterns of CoxP and Fe-CoxP NCs; XPS spectra of (c) Co 2p and (d) P 2p performed on CoxP and Fe-CoxP NCs. After phosphorization process, the Co3O4 and Fe-Co3O4 NCs are converted into CoxP and Fe-CoxP NCs, repectively. Figure 6a depicts typical SEM image of Fe-CoxP NCs, it can be seen that the original conical framework retains well. The compositions of Fe-CoxP NCs are characterized by EDS (Figure S12), indicating that Fe, Co, and P elements are uniformly distributed throughout the whole NC. The crystal structure of Fe-CoxP NCs are further analyzed by XRD (Figure 6b), both crystalline CoP phases (JCPDF no. 29–0497) and Co2P phase (JCPDF no. 32–0306) can be identified. It is notable that the intensity of peak corresponding to (121) plane of Co2P significantly increases after Fe incorporation, demonstrating that Fe incorporation is beneficial to the growth of Co2P. The HRTEM image (Figure S13) reveals that the fringes with


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lattice spacing of 2.47 Å and 2.72 Å can be attributed to the (111) planes of CoP and Co2P, respectively. Moreover, X-ray photoelectron spectroscopy (XPS) is utilized to study the composition evolution. Incorporation of Fe leads to the peak of Co 2p3/2 (778.6 eV) for Fe-CoxP NCs is negatively shifted to compared with the one in CoxP NCs (Figure 6c), indicating that the charge state of cobalt atom is distinct.[60–62] The high-resolution P 2p region for CoxP (Figure 6d) with two peaks at 130.5 and 129.6 eV are assigned to the binding energy of P 2p1/2 and P 2p3/2, respectively, and the other peak at 134 eV can be assigned to oxidized phosphate species (POx).[63–65] The peaks of P 2p3/2 (129.4 eV) and POx (133.5 eV) in Fe-CoxP also show a negative shift compared with the corresponding peaks in CoxP. Such negative shift suggests strong electron interactions between Fe and Co, suggesting Fe incorporation can modulate electronic environments of the Co centers in Fe-CoxP and promoting the HER catalysis.[66–67] For comparison, a series of Fe-CoxP NCs and FeP NPs have been prepared via conversion reaction from their corresponding Fe-Co PBA NCs and Fe-Fe PBA NPs precursors, respectively (see Supporting Information for preparation details). Theirs morphology, structure, and composition are investigated (Figure S14, S15 and S16). The result of ICP-MS (Table S1) shows that the concentration of Fe dopants increases with the concentration of [Fe(CN)6]3− anions in the anion exchange process.



Figure 7. (a) Polarization curves for various catalysts, and Fe-CoxP NCs after 1000 CV cycles in 0.5 M H2SO4; (b) chronoamperometric response curve of Fe-CoxP NCs at the overpotential of 125 mV; (c) Tafel plots for various catalysts; (d) capacitive currents as a function of scan rate for CoxP, Fe-CoxP, Fe-CoxP–1 and Fe-CoxP–2 NCs. Figure 7a displays the polarization curves of the as-prepared samples in 0.5 M H2SO4, along with the commercial Pt/C for reference. While reaching a current density of 10 mA cm–2, the overpotentials required for Fe-CoxP NCs is 127 mV, which is much smaller than those for CoxP NCs (209 mV), Fe-CoxP–1 NCs (192 mV), Fe-CoxP–2 NCs (202 mV) and FeP NPs (172 mV). After 1000 scanning cycles, the polarization curve of Fe-CoxP NCs shows no significant decay in current density, also suggesting the good durability. Moreover, chronoamperometric measurement (Figure 7b) shows only a little degradation after 200 min, indicating a relatively stable trace of HER


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activity. As shown in Figure 7c, the Tafel slopes for CoxP, Fe-CoxP, Fe-CoxP–1, Fe-CoxP–2 NCs and FeP NPs are 70, 55, 57, 59 and 68 mV dec-1, respectively. The lowest overpotential and smallest Tafel slope for Fe-CoxP NCs imply its superior HER activity. To understand the influence of Fe doped on HER performance, electrochemical double-layer capacitance (Cdl) achieved from cyclic voltammetry test are used to estimate the electrochemical surface area (ECSA) of CoxP and Fe-CoxP NCs.[68] Figure 7d indicates that the Cdl of Fe-CoxP NCs is larger than those of CoxP, Fe-CoxP–1, Fe-CoxP–2, suggesting better ion accessibility for electrolyte and larger ECSA in Fe-CoxP. The enhanced HER performance of Fe-CoxP NCs may be caused by two reasons: (1) the increase of ECSA; (2) Fe dopant can leads to more optimal free energy of hydrogen adsorption on Co sites.[69]

Conclusions In summary, a novel intercalation reaction synthesis route based on layer structured Co(OH)2 NCs as self-sacrificing templates for fabricating Fe-Co PBA NCs at ambient temperature

is

developed.

First,

[Fe(CN)6]3−

anions are

intercalated and

homogeneously anchored in the layer structured Co(OH)2 NCs matrix, which establishes the foundation for realizing the highly uniform of Fe doped at the atomic level. Then, intercalated [Fe(CN)6]3− anions reacts with adjacent Co(OH)2 host layers to form more stable Fe-Co PBA NPs. Notably, the Fe-Co PBA NPs are embedded in Co(OH)2 host layers, which can efficiently prevent the aggregation and excessive growth of Fe-Co PBA NPs. Remarkably, by simple calcination and phosphorization



process, the obtained Fe-Co PBA NCs can be further transformed into Fe-CoxP NCs while retaining a conical feature. The Fe-CoxP NCs with a significant enhancement in HER catalytic activity. This study provides a simple and effective method to realize heteroatom doping. Besides, this strategy can possibly be extended to the synthesis of other PBA material with special structure and novel morphology, which can be served as a promising platform for diverse applications, especially in the field of energy storage and conversion.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Experimental section, HRTEM image, Optical photograph, Low-magnification SEM image, SAED pattern, TGA, FT-IR spectroscopy, EDS images, and ICP-MS data.

Acknowledgements This work is supported by National Natural Science Foundation of China (no. 51407098, 51672146), and State Key Laboratory of Electrical Insulation and Power Equipment (no. EIPE17205).

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A facile and straightforward flow synthesis strategy was developed to synthesize Fe-Co PBA NCs.


Intercalation Synthesis of Prussian Blue Analogue Nanocone and

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