⇦Chinese Physics Letters, 2020, Vol. 37, No. 5, Article code 058201 Ultrafine Mo-Doped Co$_{2}$P Nanorods Anchored on Reduced Graphene Oxide as Efficient Electrocatalyst for the Hydrogen Evolution Reaction * Yi-Xuan Wang (王艺璇)1, Qing Yang (杨青)1, Chuang Liu (刘闯)1, Guang-Xia Wang (王光霞)2, Min Wu (武敏)1, Hao Liu (刘豪)1, Yong-Ming Sui (隋永明)1, Xin-Yi Yang (杨新一)1** Affiliations 1State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012 2School of Applied Physics and Materials, Wuyi University, Jiangmen 529020 Received 27 November 2019, online 25 April 2020 *Supported by the National Natural Science Foundation of China (Grant Nos. 11874027, 11774124, and 11504126) and the China Postdoctoral Science Foundation (Grant Nos. 2019T120233 and 2017M621198).
**Corresponding author. Email: yangxinyi@jlu.edu.cn Citation Text: Wang Y X, Yang Q, Liu C, Wang G X and Wu M et al 2020 Chin. Phys. Lett. 37 058201    Abstract One-dimensional (1D) transition metal phosphides (TMPs) with large specific surface areas, high charge transfer efficiency and excellent electrical conductivity have attracted significant attention in hydrogen evolution reaction (HER) as versatile and active catalysts. Herein, the sub-4 nm Mo-Co$_{2}$P ultrafine nanorods (NRs) anchored on reduced graphene oxide (rGO) were successfully synthesized by a colloidal mesostructured strategy. Electrochemical test results reveal that the Mo-Co$_{2}$P@rGO electrode exhibits superior activity with overpotentials of 204 mV and Tafel slope of 88 mV/dec for HER at 10 mA/cm$^{2}$, relative to the Co$_{2}$P@rGO electrode in 0.5 M H$_{2}$SO$_{4}$ solution. This improvement could be ascribed to the Mo doping, which results in more active sites, higher electrical conductivity and faster electron-transfer rates. This versatile strategy will provide a promising pathway for transition metal-doped compounds as an efficient catalyst. DOI:10.1088/0256-307X/37/5/058201 PACS:82.45.Yz, 78.67.Qa, 61.46.Km © 2020 Chinese Physics Society Article Text One-dimensional (1D) nanomaterials—including nanowires, nanorods (NRs) and nanotubes, and so on—are attracting much attention in fundamental research and practical applications—such as electronic and optical nanodevices, biological sensors, catalytic and energy fields.[1–5] In particular, 1D nanomaterials have a huge advantage in the hydrogen evolution reaction (HER) owing to the shortened electron transport path and increased contact areas between the active sites and electrolyte solutions.[6] Water electrolysis is a mature technique for hydrogen production, but it requires efficient electrocatalysts for the HER to achieve high current density at a low overpotential.[7] Electrocatalysts with good activity for hydrogen production are based on noble metals such as platinum (Pt); however, their serious scarcity and high cost limit their large-scale applications.[8–10] Thus, it is urgent to develop high activities, economical and efficient catalysts for HER using non-precious metal catalysts. Transition metal phosphides (TMPs) have good quasi-metallic characteristics of conductivity, which is an outstanding property for electrochemical applications.[11] Among TMPs, Co phosphides display potential applications in HER due to their advantages of excellent catalytic activity and durability.[12–15] Anion and cation doping provides a significant method to improve the catalytic performance of Co phosphides.[16,17] For instance, Guan et al. demonstrated that Mo-CoP hollow nanoarrays exhibited an overpotential of 40 mV in 0.5 M H$_{2}$SO$_{4}$ solution at 10 mA/cm$^{2}$, which is lower than that of CoP (160 mV).[17] Fe-CoP/Ti exhibited high HER performances with overpotentials of 78 mV at 10 mA/cm$^{2}$, which is superior to the CoP/Ti (128 mV).[18] Moreover, it is found that design of the advanced nanostructures (e.g., NRs, nanowires, nanotubes) is another efficient strategy to improve the HER performance.[19,20] For example, building 1D ultrafine NRs could be widely used as efficient catalysts due to their large number of active sites and excellent electron transport performance.[6] Despite the recent boom of interest in finding various efficient catalysts, it remains a hot topic to improve the catalytic performance of materials by designing advanced ultrafine nanostructures and doping with cations or anions. In this Letter, we report a facilely controlled synthesis of ultrafine Mo doping Co$_{2}$P NRs with sub-4 nm diameter attached onto reduced graphene oxide (abbreviated as Mo-Co$_{2}$P@rGO) via a colloidal mesostructured strategy. Electrochemical test results illustrate that the Mo-Co$_{2}$P@rGO NRs exhibit catalytic performances with overpotentials of 204 mV for HER to a 10 mA/cm$^{2}$ and the corresponding Tafel slopes of 88 mV/dec, which are superior to the Co$_{2}$P@rGO NRs. The excellent catalytic activity is attributed to the ultrafine nanostructures exposing a large number of active sites, providing a sufficient interface contact area with the electrolyte, and improving the electrical conductivity through the doping of Mo.[21–23] This study paves the way to in large scale obtain highly active and durable phosphide electrocatalysts for practical clean-energy devices. The ultrafine NRs have a large length-to-diameter ratio, exposing more sites to the electrolyte as electrocatalysts.[19] We successfully prepared the ultrafine nanostructures of Co$_{2}$P via a colloidal mesostructural strategy and used a similar strategy for the synthesis of Mo-Co$_{2}$P ultrafine NRs, as shown in Fig. 1. Briefly, ultrathin NRs were grown after injecting trioctylphosphine solution into a Co-Mo-oleylamine (OLA) precursor (see experimental section for more details). When the Mo-Co$_{2}$P nuclei were formed, OLA molecules protected the Mo-Co$_{2}$P nucleus from forming agglomerates because of their well-organized blocking layers on the nanocrystal surfaces.[24] Then, it served as a highly confined and high-aspect ratio template for further growth of Mo-Co$_{2}$P NRs. In order to promote the HER capacity of the catalysts, we improved the engineering of the ultrafine Mo-Co$_{2}$P NRs by coupling with rGO. The nanocarbon-based materials can be used as appropriate conductive substrates owing to their high surface area and excellent electrical conductivity.[25,26] The Mo-Co$_{2}$P@rGO catalysts have the Brunauer–Emmett–Teller (BET) specific area of 67.8 m$^{2}$/g (Fig. S1 in Supplemental Material), indicating the abundant exposed active sites and high electrocatalytic performance for the HER.
cpl-37-5-058201-fig1.png
Fig. 1. Schematic diagrams for the preparation of the ultrafine Co$_{2}$P and Mo-Co$_{2}$P@rGO NRs.
cpl-37-5-058201-fig2.png
Fig. 2. ADXRD pattern of the ultrafine Mo-Co$_{2}$P NRs, in which black lines, red circles, green lines and blue bars present the calculated pattern, the observed pattern, the difference between the observed and calculated plots, and the positions of the Bragg reflections, respectively.
The angle dispersive synchrotron x-ray diffraction (ADXRD) pattern displays the crystal structure of Mo-Co$_{2}$P NRs (Fig. 2), which performed at room temperature with an incident monochromatic wavelength of 0.6199 Å.[27] All the diffraction peaks of Mo-Co$_{2}$P can be well refined with the Co$_{2}$P phase (JCPDS No. 32–306) with space group Pnma ($a = 5.646$ Å, $b = 6.609$ Å, and $c = 3.513$ Å) consistent with our previous report.[28] In addition, these wide diffraction peaks also indicate the small grain size of prepared Mo-Co$_{2}$P NRs.
cpl-37-5-058201-fig3.png
Fig. 3. Morphological features of the synthesized Mo-Co$_{2}$P NRs: (a) TEM image of the Mo-Co$_{2}$P NRs. (b) The diameter distribution of ultrafine Mo-Co$_{2}$P NRs. [(c), (d)] Low-magnification and high-magnification HRTEM images of the Mo-Co$_{2}$P NRs. The corresponding FFT pattern was inserted in (d).
cpl-37-5-058201-fig4.png
Fig. 4. Morphological features of the synthesized Mo-Co$_{2}$P@rGO NRs: (a) Low-magnification, and (b) high-magnification TEM images of the Mo-Co$_{2}$P@rGO catalysts. (c) HRTEM image of the Mo-Co$_{2}$P@rGO catalysts. The corresponding FFT pattern was inserted in (c). (d) STEM elemental-mapping images of the Mo-Co$_{2}$P@rGO catalysts.
Morphological details of Co$_{2}$P and Mo-Co$_{2}$P NRs were characterized by transmission electron microscopy (TEM). As shown in Fig. S2, the product consists of uniform Co$_{2}$P NRs ($\sim $4 nm $\times 65$ nm) in a high yield close to 100%, which is consistent with our previous report.[28] The high uniformity of the as-prepared Mo-Co$_{2}$P NRs exhibits diameters of sub-4 nm and lengths of $\sim 71$ nm (Fig. 3(a)–3(c)), which indicates that doping of Mo can still maintain the structure of ultrafine NRs. The high-resolution TEM (HRTEM) image in Fig. 3(d) illustrates that the interplanar distance is about 0.17 nm, corresponding to the (002) facet of Mo-Co$_{2}$P NRs, which is consistent with the corresponding fast Fourier transform (FFT) pattern (inset of Fig. 3(d)). After loading with the rGO sheets, the ultrafine Mo-Co$_{2}$P NRs are uniformly dispersed on the flexible rGO framework (Fig. 4(a)). Figure 4(b) shows that the ultrafine NRs have no aggregation after attached onto rGO. It is worth noting that the Mo-Co$_{2}$P NRs maintained the orthorhombic structure with ultrathin shape, as shown in Fig. 4(c). The EDX-mapping (Fig. 4(d)) proves the existence of Co, P and Mo elements in the sample, clearly indicating that all the elements have a homogeneous distribution over the entire Mo-Co$_{2}$P@rGO catalysts.
cpl-37-5-058201-fig5.png
Fig. 5. XPS spectra of Co $2p$ (a), P $2p$ (b), and Mo $3d$ (c) of the Mo-Co$_{2}$P@rGO catalysts.
X-ray photoelectron spectroscopy (XPS) was carried out to further reveal the chemical compositions and valence states of Mo-Co$_{2}$P@rGO catalysts. As revealed in Fig. 5(a), the sharp peaks at 778.4 and 793.4 eV can be assigned to the Co $2p_{3/2}$ and Co $2p_{1/2}$ energy levels, and the peaks located at 781.7 eV and 797.5 eV can be attributed to the surface oxidation of Co$_{2}$P. The small satellites can also be observed at about 786.9 and 803.2 eV (marked by s), respectively.[29] Compared to the Co $2p$ spectra in bare Co$_{2}$P@rGO, the binding energy is positively shifted by $\sim $0.9 eV, which indicates that the electronic structure of Co centres has been modified through Mo doping.[17] Figure 5(b) shows the P $2p$ spectra of Mo-Co$_{2}$P@rGO catalysts, which contain P $2p_{3/2}$ (129.4 eV), P $2p_{1/2}$ (130.2 eV) and surface oxidized P–O species (133.3 eV).[29] As shown in Fig. 5(c), the peaks at 234 eV and 235.6 eV agree well with Mo 3$d_{3/2}$ and those at 230.8 eV and 232.5 eV correspond to Mo 3$d_{5/2}$. Two peaks of Mo $3d_{3/2}$ (235.6 eV) and Mo $3d_{5/2}$ (232.5 eV) indicate the main presence of Mo is in $+$6 state (Mo$^{6+}$), while the rest two peaks imply the presence of Mo$^{4+}$.[17,23] The HER activities of Mo-Co$_{2}$P@rGO catalysts were tested by linear sweep voltammetry (LSV) in the 0.5 M H$_{2}$SO$_{4}$ solution using a typical three-electrode system. Generally speaking, the lower overpotential ($\eta$) corresponds to higher HER activity at the same current density ($j_{\rm A}$).[30] As shown in Fig. 6(a), the Mo-Co$_{2}$P@rGO (0.02 M/1 M) electrode has the highest catalytic activity among all the Mo-doped Co$_{2}$P@rGO electrodes. The LSV curve of the commercial Pt/C presents excellent HER catalytic activity, as shown in Fig. 6(b). The Mo-Co$_{2}$P@rGO (0.02 M/1 M) catalysts exhibit only an overpotential of 204 mV, which is significantly lower than the Co$_{2}$P@rGO electrode (264 mV) at 10 mA/cm$^{2}$ in the 0.5 M H$_{2}$SO$_{4}$ solution. Therefore, we propose that Mo doping is of great importance for improving the catalytic performance. The improved HER activity of Mo-doped Co$_{2}$P should be resulted from a strengthened H–P bond and a reduced thermo-neutral hydrogen adsorption free energy ($\Delta G_{\rm H^\ast }) $[17] Tafel plots derived from LSV curves can evaluate the HER mechanism. The smaller Tafel slope suggested that smaller overpotential was needed to gain the same current density, indicating the quicker charge transfer kinetics.[31,32] The HER mechanism can be described by two possible categories: one is the Volmer–Heyrovsky mechanism and the other is the Volmer–Tafel mechanism. $$\begin{align} &{\rm H} + e^{-}\to {\rm H}_{\rm ad},~~~{\rm (Volmer)}\\ &{\rm H}_{\rm ad} +{\rm H}+ e^{-}\to {\rm H}_{2},~~~{\rm (Heyrovsky)}\\ &{\rm H}_{\rm ad} + {\rm H}_{\rm ad}\to {\rm H}_{2}. ~~~{\rm (Tafel)} \end{align} $$ Theoretically, Tafel slopes of 120 mV/dec and 30 mV/dec can be observed for the Volmer–Heyrovsky mechanism and the Volmer–Tafel mechanism, respectively.[33] Figure 6(c) presents the estimated Tafel data of the commercial Pt/C, Co$_{2}$P@rGO and the Mo-Co$_{2}$P@rGO catalysts, which are derived from the data in Fig. 6(b). According to the Tafel equation ($\eta = b \log j_{\rm A} + a$, and $b$ is the Tafel slope), the Tafel slope of the Mo-Co$_{2}$P@rGO (0.02 M/1 M) electrode is 88 mV/dec, which is lower than that of the Co$_{2}$P@rGO electrode (102 mV/dec). It indicates that the Mo-Co$_{2}$P@rGO (0.02 M/1 M) electrode exhibits the fast kinetics and excellent catalytic activity. Additionally, the hydrogen evolution process catalyzed by these two catalysts can be ascribed to the Volmer–Heyrovsky mechanism. Electrochemical impedance spectroscopy (EIS) was performed to explore the electrode kinetics during the HER process. Figure 6(d) shows the Nyquist plots and the corresponding equivalent circuits. The equivalent circuit consists of the solution resistance ($R_{\rm s}$), constant phase element (CPE), and charge transfer resistance ($R_{\rm ct}$). The diameters of the semicircles are related to the charge-transfer $R_{\rm ct}$ in the charge-transfer process.[34,35] The smaller diameter of semicircle triggers the lower $R_{\rm ct}$ value and the faster reaction rate. It manifests that Mo-Co$_{2}$P@rGO (0.02 M/1 M) catalysts acquire the lower $R_{\rm ct}$ value than Co$_{2}$P@rGO catalysts, which implies that the Mo-Co$_{2}$P@rGO has faster electron-transfer rates and catalytic kinetics than Co$_{2}$P@rGO. It also indicates that Mo doping can improve the conductivity of the composite. Finally, we investigated the stability of the Mo-Co$_{2}$P@rGO catalytic under continuous cyclic voltammetric (CV) sweeps. After 200-CV cycles, the catalysts exhibited similar curves to the initial cycle with slight anodic current loss at a certain overpotential (Fig. S3). This result reflects the good stability in a long-term electrochemical process of the Mo-Co$_{2}$P@rGO catalysts.
cpl-37-5-058201-fig6.png
Fig. 6. (a) LSV curves of the Mo-Co$_{2}$P@rGO catalysts in the different concentrations of MoCl$_{5}$/CoCl$_{2}$ (M/M). (b) LSV curves of Co$_{2}$P@rGO, Mo-Co$_{2}$P@rGO (0.02 M/1 M) and Pt/C catalysts in 0.5 M H$_{2}$SO$_{4}$ at 10 mV/s. (c) Corresponding Tafel plots in (b). (d) Nyquist plots of EIS of the Co$_{2}$P@rGO and Mo-Co$_{2}$P@rGO (0.02 M/1 M) catalysts in 0.5 M H$_{2}$SO$_{4}$.
In summary, we have successfully fabricated ultrafine Mo-Co$_{2}$P@rGO and Co$_{2}$P@rGO NRs with sub-4 nm diameters via a facile colloidal mesostructured method. Our results indicate that the Mo-Co$_{2}$P@rGO electrode exhibits superior activity with overpotentials of 204 mV for HER at 10 mA/cm$^{2}$, relative to the Co$_{2}$P@rGO electrode in 0.5 M H$_{2}$SO$_{4}$ solution. Moreover, the Tafel slope is the lowest with 88 mV/dec among the test samples. The excellent properties are derived from the ultrafine nanostructures promoting active sites full contact with electrolyte, and improving the electrical conductivity through the doping of Mo. This work points out new directions in the design and development of Mo-doped TMPs NRs as attractive catalyst materials for use in many applications. This work was performed at the BL15U1 beamline, Shanghai Synchrotron Radiation Facility (SSRF). References Surface-Induced Enhancement of Piezoelectricity in ZnO NanowiresUV–vis Absorption and PL Properties of Self-Assembled Silicon NanotubesDifferent Charging-Induced Modulations of Highest Occupied Molecular Orbital Energies in Fullerenes in Comparison with Carbon Nanotubes and Graphene SheetsFacile fabrication of PdRuPt nanowire networks with tunable compositions as efficient methanol electrooxidation catalystsExcellent energy storage performance and thermal property of polymer-based composite induced by multifunctional one-dimensional nanofibers oriented in-plane directionControllable synthesis of Co2P nanorods as high-efficiency bifunctional electrocatalyst for overall water splittingMetal Doping Effect of the M–Co 2 P/Nitrogen-Doped Carbon Nanotubes (M = Fe, Ni, Cu) Hydrogen Evolution Hybrid CatalystsIn situ growth of different numbers of gold nanoparticles on MoS 2 with enhanced electrocatalytic activity for hydrogen evolution reactionPorous carbon coupled with an interlaced MoP–MoS2 heterojunction hybrid for efficient hydrogen evolution reactionPhase-controlled synthesis and the phase-dependent HER and OER performances of nickel selenide nanosheets prepared by an electrochemical deposition routeTransition metal phosphide hydroprocessing catalysts: A reviewRecent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water SplittingCobalt phosphide microsphere as an efficient bifunctional oxygen catalyst for Li-air batteriesEnhanced Photocatalytic Hydrogen Evolution of NiCoP/g-C 3 N 4 with Improved Separation Efficiency and Charge Transfer EfficiencyHighly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide NanoparticlesMn Doping of CoP Nanosheets Array: An Efficient Electrocatalyst for Hydrogen Evolution Reaction with Enhanced Activity at All pH ValuesHollow Mo-doped CoP nanoarrays for efficient overall water splittingFe-Doped CoP Nanoarray: A Monolithic Multifunctional Catalyst for Highly Efficient Hydrogen GenerationMetallic Co 2 P ultrathin nanowires distinguished from CoP as robust electrocatalysts for overall water-splittingComponent-Controllable WS 2(1– x ) Se 2 x Nanotubes for Efficient Hydrogen Evolution ReactionFlower-like CoP microballs assembled with (002) facet nanowires via precursor route: Efficient electrocatalysts for hydrogen and oxygen evolutionEngineering a nanotubular mesoporous cobalt phosphide electrocatalyst by the Kirkendall effect towards highly efficient hydrogen evolution reactionsMo‐Doped Cobalt Phosphide Nanosheets for Efficient Hydrogen Generation in an Alkaline MediaSolution-Phase Synthesis and Characterization of Single-Crystalline SnSe NanowiresCoP nanoparticles deposited on reduced graphene oxide sheets as an active electrocatalyst for the hydrogen evolution reactionCoP nanosheet assembly grown on carbon cloth: A highly efficient electrocatalyst for hydrogen generationUltrafine Co 2 P nanorods wrapped by graphene enable a long cycle life performance for a hybrid potassium-ion capacitorStructure-optimized CoP-carbon nanotube composite microspheres synthesized by spray pyrolysis for hydrogen evolution reactionRecent advances in heterogeneous electrocatalysts for the hydrogen evolution reactionElectronic structure and d-band center control engineering over M-doped CoP (M = Ni, Mn, Fe) hollow polyhedron frames for boosting hydrogen productionRecent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reactionComponent-dependent electrocatalytic activity of PdCu bimetallic nanoparticles for hydrogen evolution reactionPolymer-Embedded Fabrication of Co 2 P Nanoparticles Encapsulated in N,P-Doped Graphene for Hydrogen GenerationCoP nanoparticles anchored on N,P-dual-doped graphene-like carbon as a catalyst for water splitting in non-acidic media
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