Molybdenum Phosphide

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Nanohybrid of carbon quantum dots/molybdenum phosphide nanoparticle for efficient electrochemical hydrogen evolution in alkaline medium Linjie Zhang, Yanmei Yang, Muhammad Asad Ziaee, Kanglong Lu, and Ruihu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00211 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Materials & Interfaces

Nanohybrid of carbon quantum dots/molybdenum phosphide nanoparticle for efficient electrochemical hydrogen evolution in alkaline medium Linjie Zhang, Yanmei Yang, Muhammad Asad Ziaee, Kanglong Lu and Ruihu Wang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, PR China. E-mail: [email protected] ABSTRACT The exploration of highly efficient non-noble metal electrocatalysts for hydrogen evolution reaction (HER) under alkaline conditions is highly imperative but still remains a great challenge. In this work, nanohybrid of carbon quantum dots and molybdenum phosphide nanoparticle (CQDs/MoP) has been firstly demonstrated as an efficient alkaline HER electrocatalyst. The CQDs/MoP nanohybrid is readily prepared through a charge-directed self-assembly of CQDs with phosphomolybdic acid (H3PMo12O40) at the molecular level, followed by a facile phosphatizing at 700 °C. The introduction of CQDs greatly helps to alleviate the agglomeration and surface oxidation of MoP nanoparticles, and ensures each MoP nanoparticle to be electronically addressed, thus significantly enhancing the intrinsic catalytic activity of MoP. The optimized CQDs/MoP exhibits high-efficiency synergistic catalysis toward HER in 1 M KOH electrolyte with a low onset potential of −0.08 V and a small Tafel slope of 56 mV dec−1 as well as high durability with negligible current loss for at least 24 h. KEYWORDS:

carbon

quantum

dots,

polyoxometalate,

electrocatalysis, hydrogen evolution reaction 1

ACS Paragon Plus Environment

molybdenum

phosphide,

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INTRODUCTION With ever-increasing consumption of fossil fuels and concerns for worsening environmental impact, considerable efforts have been devoted to searching for sustainable, benign and renewable energy sources.1, 2 Among several environmentally attractive fuels, hydrogen, a clean energy carrier with zero carbon content, is considered to be the most promising solution to meet future global energy demands.3, 4 As the most feasible and efficient route for scalable hydrogen production, electrochemical hydrogen evolution reaction (HER) has consequently received immense research interests, notwithstanding an additional electrocatalyst is mandatorily requested at the cathode to lower the intrinsically high HER overpotential.5-8 To date, Pt-based materials have exhibited excellent catalytic activity for HER in the solutions of all pH values,9 however, their large-scale applications have been severely limited due to source scarcity and high price of Pt. On the other hand, the best reported performance of counter electrode catalysts for oxygen evolution reaction (OER) has been almost exclusively achieved in alkaline electrolytes,10-12 and alkali water electrolysis has already found commercial application for several decades,13-14 yet current HER kinetics in basic solutions are sluggish at least two orders of magnitude when compared with that in acidic ones.10, 15 Therefore, to get the best out of potential of alkali electrolysis it is of great significance to develop low-cost and high-performance HER electrocatalysts in alkaline medium. Transition metal phosphides (TMPs), such as Ni2P, CoP, FeP and MoP, have shown great promises in electrocatalytic HER due to their merits of reversible binding and dissociation of H2.16-18 Among them, MoP has recently attracted extensive attention because its unique electronic

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structure which resembles that of Pt.19-21 Theoretical study have also identified MoP as a promising HER electrocatalyst, suggesting that it can introduce a good “H delivery” system which attains nearly zero binding to H at a certain H coverage.22 Generally, MoP can be facilely prepared through high-temperature phosphatizing of suitable Mo-precursors. Nonetheless, during the preparation and long-term preservation, MoP usually suffers catalytic performance degradation due to particle aggregation and surface oxidation.23, 24 Thus, an efficient strategy is urgently desired to synchronously enhance the catalytic activity and stability of MoP. To reduce particle size and guarantee a high degree of dispersion of MoP particles, two pivotal criteria should be met: (i) pony-size Mo-precursors are used; (ii) conductive support is introduced to intimately anchor the Mo-precursors to ensure their even distribution and avert the aggregation during high-temperature phosphatizing. H3PMo12O40 (HPM), as a well-known polyoxometalate (POM), is a Keggin-type polynuclear molybdenum–oxo cluster compound with nanosize of ca. 1 nm,25 and can serve as an ideal molybdenum precursor for the formation of small-sized MoP particles. Moreover, the good solubility in various solvents, strong acidity and high negative charge of HPM make it easy to anchor on supports. So far, various nanocarbons, such as carbon nanotubes (CNTs) and graphene, have been used as conductive supports to link POMs on their surfaces through a range of intermolecular or covalent interactions.25-28 This allows molecular dispersion of POMs on the nanocarbons, thus alleviating the aggregation of MoP, and each MoP nanoparticle is electronically addressed and involved in energy and electron transfer processes of HER. However, the anchor on external surfaces of CNTs and graphene is still unable to well settle the stability problem of MoP due to surface oxidation during long-term 3



storage. A feasible resolution is to encircle these MoP nanoparticles by the support to form a surface layer between MoP and atmosphere. Such an ideal support is highly expected to have equivalent conductivity to CNTs or graphene, and similar size to HPM clusters in order to give rise to a molecularly integrated hybrid to protect MoP against surface oxidation. As a peculiar carbon nanomaterial, carbon quantum dots (CQDs, fragments of graphite) with size below 10 nm featuring with good aqueous solubility, favorable electronic conductivity, high stability, easy functionalization and low toxicity, are promising support for MoP.29, 30 The abundant surface functional groups, such as –OH and –COOH, enable CQD an ideal carbon support to be modified. When modified with ethylenediamine (EDA), CQDs are readily protonated in the presence of strong acidic HPM, thus HPM can be easily combined with CQDs via electrostatic interaction between the positively charged CQDs and negatively charged PMo12O403−. The combination of tiny HPM cluster anions with CQDs is expected to fabricate small and uniform CQDs/MoP nanohybrid with efficient electrocatalytic activity for HER. To best of our knowledge, there is no report on hybridizing CQDs and metal phosphides so far. Herein, we developed a facile charge-directed self-assembly strategy to combine HPM with CQDs. After a subsequent optimized phosphatizing process, the as-obtained strongly-coupled CQDs/MoP nanohybrid has demonstrated high HER electrochemical performance with a low onset potential of −0.08 V in 1 M aqueous KOH solution. The Tafel slope of CQDs/MoP is determined as 56 mV dec−1, which is comparable to that of commercial 20 wt% Pt/C (50 mV dec−1).

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EXPERIMENTAL SECTION All reagents were of analytical grade and used as-received. Graphite rods (0.6 cm × 15 cm, 99.9995%) were bought from Alfa Aesar. Potassium hydroxide (KOH), phosphomolybdic acid (H3PMo12O40·nH2O) and ethanediamine (C2H8N2, EDA) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium hypophosphite (NaH2PO2) was purchased from Aladdin (Shanghai). Ultrapure water obtained by Millipore Milli-Q system was used to prepare all solutions. Preparation of Ethanediamine-Modified CQDs (ECQDs). CQDs were synthesized based on a previously reported method.31 A hydrothermal treatment was adopted to prepare ECQDs using the modified literature method.32 Typically, ethanediamine (EDA, 0.2 mL) was added into the dark brown aqueous CQDs solution (50 mL, 1 mg mL−1), and was stirred at room temperature for 12 h. The resultant mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 105 °C for 12 h. After cooling to room temperature, ECQDs were collected by freeze-drying. Preparation of ECQDs/HPM. After the pH value of aqueous ECQDs solution (50 mL, 0.5 mg mL−1) was adjusted to ca. 7 by the dropwise addition of aqueous HCl solution (0.1 M), the aqueous H3PMo12O40 solution (125 mL, 1 mg mL−1) was added. After vigorous stirring at room temperature for 24 h, the solid was centrifugally collected, washed with deionized water for three times, and freeze-dried. The as-obtained sample is denoted as ECQDs/HPM. Preparation of CQDs/MoP. ECQDs/HPM and NaH2PO2 with a mass ratio of 1 : 10 were mixed together and ground into powder. The mixture loaded in a crucible was then transferred to a tube 5



furnace, and heated up to 700 °C with a heating rate of 3 °C min−1 in Ar atmosphere, followed by phosphatizing treatment at this temperature for 3 h. After natural cooling to room temperature, the product was washed with deionized water for three times to remove any soluble impurities and dried at 60 °C. The as-obtained product is denoted as CQDs/MoP. For comparison, CQDs/MoP-650 and CQDs/MoP-750 were also synthesized by the same procedure with that of CQDs/MoP except at different phosphatizing temperatures of 650 and 750 °C, respectively. In addition, free MoP was synthesized by directly phosphatizing of HPM. Physical Characterization. Transmission electronic microscopy (TEM) and energy dispersive X-ray (EDX) elemental mapping were performed on Tecnai G2 F20 electron microscope at an acceleration voltage of 200 kV. Fourier transform infrared spectra (FT-IR) were recorded on a Nicolet 360 spectrometer using KBr disk technique at wavenumbers ranging from 4000 to 400 cm−1. X-ray photoelectron spectroscopy (XPS) was conducted on KRATOS Axis Ultra-DLD spectrometer with monochromatised Al Kα X-ray (hν = 1486.6 eV). Calibration of the binding energy was achieved by setting the binding energy of the C 1s peak to 284.8 eV. Powder X-ray diffraction (XRD) was performed on an X-ray diffractometer (Empyrean, Holland Panalytical). Raman spectra were taken on an HR 800 Raman spectroscope (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed on a Jobin Yvon Ultima 2 system. Electrochemical Measurements. All electrochemical measurements were carried out in a conventional three-electrode cell using an electrochemical workstation (CHI760E, CH Instrument, Shanghai) with a glass carbon electrode (GCE, Φ = 5 mm) as working electrode, and 6


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graphite rod and Ag/AgCl (sat. KCl) as counter and reference electrodes, respectively. The catalyst film was prepared by dropping 10 µL homogeneous aqueous solution of CQDs/MoP ink (4 mg mL−1) onto the surface of GCE followed by coating with 5 µL of Nafion solution (0.5 wt%; DuPont) and dried in air. Polarization curves were obtained using linear sweep voltammetry (LSV) curves conducted in 1 M aqueous KOH solution with a scan rate of 10 mV s−1 and then corrected by the iR loss according to the following equation: Ecorrection = E − iR. All potentials in this work were converted to a reversible hydrogen electrode (RHE) scale using the following equation: ERHE = EAg/AgCl + E°Ag/AgCl + 0.059 × pH. For the electrochemical impedance spectroscopy (EIS) measurement, the frequency range was 106 to 1 Hz. The time-dependent catalytic current density of the catalyst during electrolysis was conducted at an overpotential of 200 mV after equilibrium. RESULTS AND DISCUSSION

Figure 1. Schematic illustration of the synthesis of CQDs/MoP nanohybrid. As shown in Figure 1, the combination of H3PMo12O40 with ECQDs (ECQDs/HPM) was achieved by a facile charge-directed self-assembly strategy. EDA was grafted onto the surface of CQDs by a ring-opening reaction between the epoxy groups on CQDs and amino groups of EDA 7



to form ECQDs. Subsequent assembly with HPM gave rise to the ECQDs/HPM complex based on strong electrostatic interaction between the positively charged ECQDs and negatively charged cluster anion of HPM. Afterwards, the ECQDs/HPM complex was phosphatized using NaH2PO2 at 700 °C under Ar atmosphere to yield the CQDs/MoP nanohybrid, in which the mass fraction of MoP is up to 68.9 wt% based on inductively coupled plasma (ICP) analysis. It should be pointed out that the modification of CQDs by EDA plays a crucial role in combination with HPM to form the ECQDs/HPM complex. Without modification, no complex was obtained from pristine CQDs. Figure S1 shows the typical TEM images of ECQDs, revealing that the ECQDs are well dispersed with particle sizes ranging from 1 to 5 nm. Besides, the minority crystal lattices (d = 0.321 nm) of the as-prepared ECQDs are observed, indicating that they are consisted of several graphene layers.31 The successful modification of CQDs by EDA is confirmed by discrepant Fourier transform infrared (FT-IR) spectra of CQDs and ECQDs (Figure S2). Compared with FT-IR spectrum of pristine CQDs, the absorption peak of C−O−C at 1230 cm−1 almost disappears in that of ECQDs, and three new peaks at 1560, 1456 and 1300 cm−1 associated with C=N, C−N and −CH2 are observed.33 Moreover, X-ray photoelectron spectroscopy (XPS) was performed to probe the chemical states of surface elements on ECQDs (Figure S3). The XPS survey spectrum of ECQDs shows the presence of N 1s peak at 400 eV besides the dominant C 1s peak at 284 eV and O 1s peak at 531 eV, respectively. In detail, the high-resolution XPS spectrum of C 1s can be deconvoluted into four peaks at 288.4, 286.4, 285.4 and 284.8 eV, corresponding to C=O/C=N, C−O/C−N, C−OH and sp2-hybridized C=C, respectively.34, 35 In high-resolution XPS spectrum of N 1s, three binding energy peaks assigned 8


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to graphitic N (N1, 400.9 eV), pyrrolic N (N2, 399.9 eV) and pyridinic N (N3, 398.9 eV) can be clearly distinguished.36, 37 Notably, more structural defective sites are introduced into ECQDs after EDA modification, as evidenced by their Raman spectra in which higher ratio of the D and G band intensities (ID/IG) of ECQDs (1.18) than that of pristine CQDs (1.07) is observed (Figure S4).38 To verify the successful combination of HPM with ECQDs, FT-IR spectra were measured. Compared with that of ECQDs, FT-IR spectrum of ECQDs/HPM shows new bands in the region of 1100 to 800 cm−1 (Figure S5). In detail, the bands at 1065, 965, 873 and 790 cm−1 can be ascribed to P−Oa, Mo−Od, Mo−Ob−Mo and Mo−Oc−Mo vibration bands respectively, which are characteristic peaks of Keggin units of H3PMo12O40,39 affirming the formation of a fully integrated compound.

Figure 2. (a) XRD pattern of CQDs/MoP. (b) TEM and (c) HRTEM images of CQDs/MoP. (d−g)

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HAADF-STEM image and elemental mappings of C−K, P−K and Mo−K of CQDs/MoP; scale bars, 50 nm. The powder X-ray diffraction (XRD) pattern of as-prepared CQDs/MoP nanohybrid shows a broad peak at 26° (Figure 2a), which refers to the amorphous carbon.31, 40 Additional dominant peaks at 27.8°, 31.1°, 43, 57°, 64.8°, 67.0° and 74° matches well with the (001), (100), (101), (110), (111), (200) and (201) lattice planes of hexagonal MoP phase (JCPDS # 24−0771), respectively. The XRD results indicate that ECQDs/HPM has been completely transformed into CQDs/MoP without impurities. Scanning electron microscopy (SEM) analysis was conducted to investigate the morphology of CQDs/MoP. It shows that the CQDs/MoP hybrids are irregular particles with a small average diameter around 20 nm (Figure S6). Noticeably, the transmission electron microscopy (TEM) further reveals that the MoP nanoparticles are well decorated by the CQDs, as disclosed by the brighter contrast of the external region with light grey color than that of the inner (Figure 2b). Due to the closely physical contact, these CQDs/MoP particles slightly heap up. However, they show good water dispersibility, thus contributing to an even distribution of catalysts ink on the electrode, which is beneficial for improvement of catalytic performance. In striking contrast, the pristine MoP without the armor of CQDs generally has a much bigger and widely distributed particle size (50−100 nm), and is severely aggregated together (Figure S7). This can also be supported by the much sharper and narrower peaks shown in the PXRD pattern of MoP than those of CQDs/MoP (Figure S8). Moreover, in a high-resolution TEM (HRTEM) image of CQDs/MoP (Figure 2c), an obvious interface between two components can be observed

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clearly. The distinct fringe intervals of 0.28 and 0.33 nm on the respective side are consistent with the d spacings of (100) and (002) lattice planes of MoP and CQDs, respectively. Besides, a fully developed graphitic carbon nanostructure adhered to MoP nanoparticle is also observed, possibly owing to the fusion of CQDs under high temperature treatment during phosphatizing. The seamless integration of CQD and MoP would enhance the catalytic performance for HER owing to their significant synergistic effects.41 From the energy-dispersive X-ray spectroscopy (EDS) analysis, it could be ascertained once again that the as-obtained phosphatized sample is CQDs/MoP (Figure S9). In addition, elemental mappings are conducted to survey the elemental distribution of C, P and Mo in CQDs/MoP. Figure 2d‒g shows the high-angel angular dark field scanning TEM (HAADF-STEM) image and corresponding elemental mapping images. As observed, the C, P and Mo signals uniformly spread over all particles, revealing the homogeneous distribution of C, P and Mo.

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Figure 3. XPS survey spectrum of CQDs/MoP, insets are the high-resolution spectra of Mo 3d and P 2p core levels. In order to determine the surface composition and chemical state of each element in CQDs/MoP, XPS characterization was further carried out, and the results are exhibited in Figure 3. The XPS survey spectrum indicates that CQDs/MoP is composed of C, O, Mo and P elements. The high-resolution XPS spectrum of Mo 3d is fitted into two spin−orbit doublets with four different signals.42 The peaks at 227.82 and 230.92 eV with a separation of 3.1 eV are assigned to partial positively charged Mo (Moδ+, 0 < δ ≤ 4) species in MoP compared with metallic Mo0 (227.6 eV).43, 44 Moreover, the remaining peaks at higher binding energies of 232.06 and 235.17 eV are indexed to MoOx species on account of the surface oxidation that could only be detected by XPS (less than 10 nm).25, 45 The measured high-resolution XPS spectrum of P 2p is fitted into three components, the peak with binding energy of 133.7 eV could be attributed to PO43− or P2O5, which is caused by surface-oxidation.46 And other two peaks with binding energies of 130.4 and 129.5 eV are ascribed to MoP.47, 48 It should be mentioned that P is negatively charged (Pδ–) here, as indicated by the lower binding energy than that of P0 (130.2 eV). The charge polarization of Moδ+ and Pδ– uncovered by XPS analysis suggests electron transfer process involved in CQDs/MoP. According to the previous reports, proton relays from pendant acid−base groups positioned close to the metal center can serve as the active sites for catalytic hydrogen evolution.49-51 Hence, it is expected that CQDs/MoP will have good HER activity. Electrocatalytic HER activity of CQDs/MoP was investigated in 1.0 M aqueous KOH solution

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with a standard three-electrode system. For comparison, the catalytic activities of pristine MoP, ECQDs and commercial Pt/C (20 wt％) were also tested under the same conditions. As shown by the polarization curves in Figure 4a, both bare glassy carbon electrode (GCE) and ECQDs exhibit negligible HER activity, while the pristine MoP catalyst shows a mediocre HER activity with an overpotential of 280 mV to achieve the current density of 20 mA cm−2. In remarkable contrast, CQDs/MoP nanohybrid demonstrates significantly improved electrocatalytic activity, which gains a low onset potential (determined at j = −0.5 mA cm-2 in the polarization curve) of −0.08 V, and demands an overpotential of only 210 mV to achieve the same current density of 20 mA cm−2, which lags behind that of benchmark Pt/C catalyst at only 91 mV (Figure 4a). The superior catalytic performance of CQDs/MoP catalyst to that of MoP is further confirmed by their Tafel slopes as shown in Figure 4b. The Tafel slope of CQDs/MoP is 56 mV dec-1, which is much lower than that of MoP (87 mV dec-1), and is almost the same as that of Pt/C (50 mV dec-1), suggesting the HER process on CQDs/MoP catalyst was promoted with the Volmer–Heyrovsky mechanism. Obviously, compared with the pristine MoP, the HER catalytic activity of CQDs/MoP nanohybrid is synergistically enhanced.

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Figure 4. (a) Polarization curves for CQDs/MoP, ECQDs, MoP, commercial Pt/C (20 wt%) and bare glassy carbon electrode (GCE) in 1 M KOH electrolyte with a scan rate of 10 mV s−1. (b) Tafel plots for CQDs/MoP, MoP and commercial Pt/C (20 wt%). (c) Nyquist plots for CQDs/MoP and MoP, inset shows the corresponding equivalent circuit model. (d) Stability of CQDs/MoP with polarization curves before and after 1000 cycles with a scan rate of 50 mV s−1 (inset: time-dependent catalytic current density curve during electrolysis for CQDs/MoP at η = 200 mV in 1 M KOH electrolyte, without IR compensation). To gain further insights into the electrode kinetics of catalysts, electrochemical impedance spectra (EIS) were measured. The EIS Nyquist plots of CQDs/MoP and MoP are composed of a semiarc at high-frequency region and a diagonal line at low-frequency region (Figure 4c). To simulate the HER kinetics of catalysts, the Nyquist plots are fitted to corresponding equivalent circuit models (inset in Figure 4c). According to the fitting results, the charge-transfer resistance

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(Rct) for CQDs/MoP is determined to be 13.7 ohm, which is lower than that of MoP (18.3 ohm). The lower Rct suggests a more efficient and rapid electron transport process on the interface between CQDs/MoP and electrolyte, further verifying the structural superiority of CQDs/MoP nanohybrid to pristine MoP.52-54 Stability is another significant concern for the wide application of HER catalyst. The durability of CQDs/MoP was first tested by continuous cyclic voltammetric (CV) sweeping between 0 and −0.3 V vs. RHE at a scan rate of 100 mV s−1 in 1 M KOH electrolyte. As shown in Figure 4d, after 1000 repeated cycles, CQDs/MoP shows very slight current density loss with a similar polarization curve to the initial one. The i–t curve during HER at a fixed overpotential of 200 mV for 24 h was also recorded to evaluate the stability of CQDs/MoP. As shown in the inset in Figure 4d, negligible current degradation is observed in 24 h. Furthermore, TEM observation further indicates that the morphology of the spent CQDs/MoP remains unchanged after the long-term reaction (Figure S10), suggesting the CQD moiety can effectively reduce the disintegration tendency of the strongly coupled CQDs/MoP during the accelerated durability test. All these results confirm the efficient catalytic activity and excellent stability of CQDs/MoP nanohybrid. We thereafter summarize a comparison of HER catalytic performance of CQDs/MoP with several other earth-abundant transition-metal-based catalysts, the results further testify the high performance of CQDs/MoP (Table S1).

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Figure 5. (a) Polarization curves for CQDs/MoP-650, CQDs/MoP and CQDs/MoP-750 in 1 M KOH electrolyte with a scan rate of 10 mV s−1. (b) Tafel plots for CQDs/MoP-650, CQDs/MoP and CQDs/MoP-750 derived from their corresponding polarization curves shown in panel (a). (c) Comparison for the catalytic activities of CQDs/MoP-650, CQDs/MoP and CQDs/MoP-750. (d) EIS for CQDs/MoP-650, CQDs/MoP and CQDs/MoP-750, inset is the Nyquist plots in high-frequency region. The influence of phosphatizing temperature on catalytic activity of CQDs/MoP was also explored. Contrasting to CQDs/MoP, CQDs/MoP-650 and CQDs/MoP-750 were successfully prepared at temperatures of 650 and 750 °C, respectively (Figure S11). Their electrocatalytic performances toward HER are compared and the results are shown in Figure 5 and Table S2. The polarization curves disclose that their catalytic activities follow the trend: CQDs/MoP > CQDs/MoP-750 > CQDs/MoP-650 (Figure 5a). Their corresponding Tafel plots with the linear

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regions fitted into the Tafel equation are then compared. As shown in Figure 5b, the Tafel slope of CQDs/MoP (56 mV dec−1) is much lower than those of CQDs/MoP-650 (115 mV dec−1) and CQDs/MoP-750 (78 mV dec−1). A histogram visually comparing the Tafel slopes and HER onset potentials further discloses the fastest reaction kinetics of CQDs/MoP (Figure 5c). Moreover, Nyquist plots obtained from EIS measurements of the samples are given in Figure 5d. The fitting results reveals that CQDs/MoP possesses the lowest charge transfer resistance of 13.7 ohm among the series, suggesting the most efficient electron transport process on the interface between CQDs/MoP and electrolyte. All the above results prove that 700 °C is the optimized phosphatizing temperature, either increase or decrease of the temperature causes a declined catalytic performance for CQDs/MoP. It is deduced that low temperature leads to the formation of MoP nanoparticles with poor crystallinity (Figure S11), while high temperature generates particles with large sizes due to severe aggregation of MoP nanoparticles. This assumption is confirmed by the microscopic observations. As seen from their typical SEM and TEM images (Figure S12), CQDs/MoP-650 exhibits slightly smaller and better dispersive particles than CQDs/MoP-750. However, the crystalline grains are not quite distinct. In contrary, CQDs/MoP-750 has discernable crystallites, but most of them are severely gathered together. CONCLUSIONS A facile charge-directed self-assembly strategy is designed to combine CQDs with H3PMo12O40 to form the ECQDs/HPM complex precursor, and through a subsequent phosphatizing process the highly dispersive and small-sized CQDs/MoP nanohybrid is successfully generated for the

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first time. The seamless integration of CQDs and MoP nanoparticles in CQDs/MoP nanohybrid exerts significant synergistic effects toward HER. The charge-polarized pairs of Moδ+ and Pδ– in MoP serve as the main active sites, while the CQDs contribute to alleviate the agglomeration of MoP and enable each MoP nanoparticle to be electronically addressed during HER, thus boosting the intrinsic activity of MoP. Compared with pristine MoP and CQDs, the catalytic performance for HER of CQDs/MoP in the alkaline medium is greatly enhanced, featuring with much reduced onset overpotential and Tafel slope, as well as strong stability for at least 24 h, which approach to those of benchmark Pt/C catalyst. In summary, this study opens up an exciting avenue to the design and fabrication of efficient electrocatalysts based on CQDs–TMPs composites by integrating their respective merits. ASSOCIATED CONTENT Supporting Information. Figure S1‒S12 and Table S1, S2. Supplementary materials structural characterizations, e.g. SEM and TEM images, FT-IR spectra, XPS, Raman spectra, PXRD and EDS spectrum.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (R. W.)

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS 18


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Graphic Abstract Nanohybrid of carbon quantum dots/molybdenum phosphide nanoparticle for efficient electrochemical hydrogen evolution in alkaline medium Linjie Zhang, Yanmei Yang, Muhammad Asad Ziaee, Kanglong Lu and Ruihu Wang*

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Molybdenum Phosphide

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