In Situ Time Resolved-XAFS and SAXS Revealed an Unexpected

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C: Physical Processes in Nanomaterials and Nanostructures

In Situ Time Resolved-XAFS and SAXS Revealed an Unexpected Phase Structure Transformation During the Growth of Nickel Phosphide Nanoparticles Yuanyuan Tan, Dongbai Sun, Hongying Yu, Shuqiang Jiao, Yu Gong, Shi Yan, Zhongjun Chen, Xueqing Xing, Guang Mo, Quan Cai, and Zhonghua Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03883 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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The Journal of Physical Chemistry

In Situ Time Resolved-XAFS and SAXS Revealed an Unexpected Phase Structure Transformation during the Growth of Nickel Phosphide Nanoparticles Yuanyuan Tan†, Dongbai Sun‡, Hongying Yu‡*, Shuqiang Jiao†*, Yu Gong§, Shi Yan⸸, Zhongjun Chen§, Xueqing Xing§, Guang Mo§, Quan Cai§, Zhonghua Wu§* †State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, P. R. China; ‡School of Materials Science and Engineering, Sun Yat-Sen University, P. R. China; ⸸Laboratory of Microfabrication, Institute of Physics, Chinese Academy of Science, Beijing, P. R. China; §Institute of High Energy Physics, Chinese Academy of Sciences, & University of Chinese Academy of Sciences, Beijing, P. R. China.

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ABSTRACT

Nickel phosphide (Ni-P) nanoparticles have been extensively studied for their promising catalytic activities in both hydrogen evolution reactions (HER) and oxygen evolution reaction (OER), however, controllable synthesis of nickel phosphide catalyst is still a challenge for their complex phase transformations during synthesis procedure. Deep understandings of nanoparticles formation mechanism should be taken into account for an efficient catalyst tailor of size, shape, structure, and eventually performance. Unfortunately, few reports were regarding to their formation processes. In this regard, we investigated the formation process of Ni-P nanoparticles by a conjunction of in situ XAFS from Ni K-edge, ex situ XAFS from P K-edge, in situ SAXS, and HRTEM techniques. A novel phase structure transformation was unraveled from views of evolutions of chemical valence, coordination structures, and size distributions along with reaction time. The results demonstrated a 4-stage formation mechanism of Ni-P nanoparticles: nucleation of crystalline Ni, nucleation of Ni-P, phase transformation from crystalline Ni to non-crystalline Ni-P, and growth of non-crystalline Ni-P nanoparticles. Furthermore, different growth manners were also observed at different growing stages. Our results shed light on efficient control of phase structure, particle size, and compositions of nickel phosphide nanoparticles, which would enlighten and promote their further development.

KEYWORDS. Phase Structure Transformation; Nickel phosphide nanoparticles; in situ XAFS; in situ SAXS; Crystalline to Non-crystalline


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INTRODUCTION

For a security concern of our energy future, the development of clean and renewable energy sources has become as an urgent issue. Increasing efforts have been put into the exploration of effective energy conversions from nature resources into high-value products, such as hydrogen. As a promising candidate for hydrogen evolution reaction (HER) catalysts, phosphide nanomaterials, such as nanoparticles,

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nanosheets,

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nanofilms,

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various nickel

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have been de-

signed and developed heading for a desirable HER catalytic activity. As it is known that the attractive performances of nanomaterials are greatly dependent on their size, shape, and structure. Consequently, it is beneficial to tailor nanomaterials with desired morphologies and microstructures with a deep understanding of their formation mechanism. Therefore, it is of great importance to explore the formation mechanism. However, researches on the formation mechanism of nickel phosphide nanomaterials is still scarce compared with noble nanoparticles.10-12 The study of nanoparticles formation mechanism could be traced as early as 1950s, 13-14 in which LaMer has given a famous and widely cited explanation for the formation of monodispersed hydrosols as burst nucleation. Besides LaMer’s work, Turkevich

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put forth an “organizer”

model for the nucleation of Au nanoparticles, in which the citrate ion took the role of the “organizer”. Then the Ostwald ripening was proposed to describe particle size evolutions with a later development of Lifshitz-Slyozow-Wagner, LSW theory. 16-17 The next headway in the study of nanoparticle formation did not come until 1997, when Finke and Watzky

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presented a

series of works on the formation mechanism of Pt, Ru, Ir, Rh, and Pd. After several years efforts, they concluded a double autocatalytic mechanism consisting of four steps: slow continuous


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nucleation, fast autocatalytic surface growth, bimolecular agglomeration, and another autocatalytic agglomeration step, moreover, a Finke-Watzky Model.23 These continuous advancements verified the importance of mechanistic studies underlying the synthesis of nanoparticles. 24-25 The elucidation of nanoparticles formation mechanisms has been realized into practice thanks to the ex- and in situ measurements developments, especially the increasingly applications of synchrotron radiation facility-based techniques. X-ray absorption fine structure spectroscopy (XAFS) has attracted much more attention due to its sensitivity on the element figure-print identification, chemical valence, and coordination structures. While, as a powerful technique for the study on nanoparticles structure, size, and shape, small angel X-ray scattering (SAXS) have also been extensively employed. For their facile applications in in situ and non-destructive ways, these techniques have been introduced to probe nanoparticles formation mechanism

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as a

guidance for effective catalyst synthesis. Furthermore, the combinations of XAFS and/or SAXS with other techniques are increasingly reported for detailed information on the growth of nanoparticles in solution. For instance, XANES and SAXS used on gold nanoparticle formation29; XAFS and TEM jointly applied in the study of nanoparticles evolution SAXS/WAXS/UV-Vis study of the nucleation and growth of nanoparticles

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;

. These reports

verified that XAFS and SAXS techniques are powerful in the exploration of nanoparticle growth. In this regard, in this study we used a conjunction of in situ XAFS, SAXS, and TEM techniques for a fully under-standing of the formation mechanism of Ni-P nanoparticles prepared by high voltage pulse discharging method in solution. The structure and chemical valence evolutions of Ni and P was elucidated by in situ XAFS from Ni K-edge and ex situ XAFS from P K-edge,


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respectively. The morphology, phase, and size changes of the particles were also directly visualized by HRTEM in an ex situ way.

EXPERIMENTS

Ni-P nanoparticles preparation method. A high voltage pulse discharge method in solution has been used for the preparation of Ni-P nanoparticles, which has been described in details in our previous report.

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Briefly, Nickel sulfate hexahydrate (NiSO4·6H2O) and sodium

hypophosphite monohydrate (NaH2PO2·H2O) were used as purchased to prepare 1 mol/L nickel– sulfate solution and 2 mol/L sodium hypophosphite solution, then the two solutions were mixed under an optimized proportion to prepare reaction solution. After the mixture was heated to 80 °C while stirring, 30 pulses with a high voltage of 1000 V were released into the solution by a pair of needle-like electrodes made from tungsten wires. The distance of the two electrodes was set as 3 mm with a discharging frequency of 20 Hz. In this synthesis, the pulse discharge played as a trigger, which started the formation process of Ni-P nanoparticles. HRTEM observations. A TEM of JEOL-2010 with a CCD model of TVIPS F114 was operated at 200 keV with a beam current of 105 µA for the observations of nanoparticles morphology at different reaction times of 30, 60, 120s, and 210s. TECNAI F20 was also used when observing the morphology of the final product non-crystalline Ni-P nanoparticles. For each TEM observation, one drop of the reaction suspension was put on the ultrathin carbon-coated copper grids. Several pieces of filter paper were placed under the copper grids to remove the remaining reaction solution. After natural evaporation of the solvent, the samples are ready for HRTEM observations.


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XAFS spectra and SAXS patterns. A schematic map of the reaction cell for in situ QXAFS and SAXS measurements is shown in Scheme 1. The cell was made of stainless steel with an X-ray pathway about 1 mm. Kapton film was used as X-ray windows. The cell was put into an aluminum made holder to control its temperature by water circulation. XAFS and SAXS measurements were conducted at Beijing Synchrotron Radiation Facility (BSRF) at a storage ring energy of 2.5 GeV and electron current of 250 mA. In situ XAFS spectra from Ni K-edge were collected at the beamline 1W2B in transmission mode at a time interval of 30s just after the pulse discharge. A double-crystal Si (111) monochromator was used to select the incident X-ray energy ranging from 8133 to 9133 eV with a resolution (∆E/E) about 2×10-4. Ex situ XAFS spectra from P K-edge was collected at beamline station 4B7 in fluorescence mode with a scanning energy range from 2100 to 2754 eV. The fluorescence signal was recorded by a Si (Li) detector. The analysis of XAFS data from both Ni and P K-edges were done by the Demeter software package (Univ. of Chicago)

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. The reference spectra of Ni and P K-edges were

calculated by using FEFF9 code (Univ. of Washington).

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Specific structural information, such

as coordination number, interatomic spacing and mean-squared disorder,

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could be determined

through fitting theoretical pathways to the experimental spectra. In situ SAXS patterns were collected at the beamline 1W2A with an incident X-ray wavelength of 1.54 Å selected by a triangle bending Si (111) crystal. The beam size was set to 1.4 × 0.2 mm2 (H × V) with a flux of 5.5 × 1011 cps. A Mar 165 two-dimensional charge coupled device (CCD) detector with 2048 × 2048 pixels was used to record the SAXS patterns. The pixel size is 79 µm. The sample-todetector distance was 1845 mm, covering a q-range of 0.09-2.24 nm-1. Here, scattering vector was calculated by q=4π·sinθ/λ, in which θ is half the scattering angle and λ is the wavelength of the incident X-ray beam. The size evolution was calculated by our home-developed programs 36-


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after data conversion from 2D scattering pattern to 1 dimensional q values proceeded by FIT

2D.38

Scheme 1. A schematic for in situ XAFS and SAXS measurements at the beamlines 1W2B and 1W2A in BSRF. The reaction cell was embedded into an alumina made temperature controller by water circulation. XAFS spectra from Ni K-edge were collected in transmission mode at a time interval of 30s, while SAXS patterns were recorded at a time interval of 10s by a Mar 165 CCD. RESUTLS AND DISCUSSION HRTEM images of the samples collected at different reaction times of 30, 60, 120, and 210s as shown in Figure 1, have displayed the morphology and structure evolutions of the growing nanoparticles. As the enlarged images shown in the left column in Figure 1, the initially formed nanoparticles observed well defined lattice fringes with a d-spacing of 0.19 nm, very close to (111) plane of crystalline FCC Ni

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and also (012) plane of crystalline NiO

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. However, the

diffraction spots after Fourier transformation indicates another d-spacing around 0.17 nm, closing to (002) plane of crystalline FCC Ni

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, but much larger than d-spacing (0.149 nm) of

(104) and (110) planes and smaller than d-spacing (0.244 nm) of (101) and (003) of NiO

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.


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Furthermore, no Ni-O or P-O bonding was observed from Ni K-edge XAFS spectra or P K-edge spectra after centrifugation of the finally obtained particles

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. Therefore, this initially obtained

nanocrystal was likely to be crystalline FCC Ni, but further evidence was needed such as coordination structures obtained from XAFS data sets. Along with the reaction time, the size of the nanoparticles has increased from abound 3 to 15 nm. Simultaneously, the lattice fringes at the edge of the nanoparticles became blurry as the increase of particle size, indicating a phase structure change from crystalline to non-crystalline. Finally, no clear lattice fringes could be observed in the particles due to a wholly phase structure transformation into non-crystalline (Figure S1), which has also been confirmed by our previous studies.

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For further illustrating

the structure change, inversed Fourier-transform pictures of these HRTEM images were performed as shown in the right column of Figure 1. For the first three samples with reaction time of 30, 60, 120s, some bright diffraction spots are scattered in the faint diffuse halation. But only a diffuse scattering ring appeared without any diffraction spots for the last sample with reaction time of 210s. These results confirm again that the first three samples have some crystalline components, while the last sample is mainly in a non-crystalline structure. However, crystalline FCC Ni structure could also be possessed by Ni-P nanoparticles with a low P content. 43

Therefore, it is difficult to identify the components and phase structures with only HRTEM

images. In this regard, XAFS techniques were employed for further investigation of the initial species during the formation process of non-crystalline Ni-P nanoparticles.


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Figure 1. HRTEM images of the growing nanoparticles at reaction times of 30, 60, 120 and 210s. As the particle size increased along with the reaction time, the initially formed nanoparticles with well-defined lattice fringes have trans-formed into a non-crystalline structure. Starting from the edge of particles, and then gradually reaching to the core of the particles. In situ QXAFS spectra from Ni K-edge with an interval of 30s were collected at beamline 1W2B in transmission mode, as shown in Figure 2. For comparison, Figure 2 (a) also included XANES spectra from standard Ni foil (6 µm) and the final non-crystalline Ni-P nanoparticles, whose noncrystalline nature has been well described in our previous reports.

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The spectrum at 0s is

corresponding to the freshly prepared reaction solutions without pulse discharge, in which the content of metallic Ni was considered as 0. Noticeably, the sharp white line peak in Figure 3 (a)


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showed a gradual decrease in intensity and a slight lower energy side shift in position along with the reaction time, which could be attributed to the decrease of averaged chemical valence of Ni.25 Specifically, the intensity decrease indicates the composition changes of Ni-containing species: the decrease of positive-charged Ni2+ ions and the increase of metallic Ni. The position shift to lower energy side also indicates a transition from positive charged state to a metallic state. When the reaction time reached to 300s, the XANES spectrum from the reaction mixture showed a feature closing to that of non-crystalline Ni-P nanoparticles, verifying the formation of noncrystalline Ni-P nanoparticles. The white line peak also shared the same energy with that of Ni

0.8 0.4 0.0 8320

8340

8360

8380

E (eV) 2+

Ni hydrates Crystal Ni Ni-P NPs

0

0 30

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0

(c)

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100 80 60 40 20 0

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8380

1.2

0

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E (eV)

Exp Fit

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8340

(b)

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8320

1.6

0

Ni-P NPs Ni foil

2.0

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0s 30s 60s 90s 120s 150s 180s 210s 240s 270s 300s

Content (%)

(a)

Intensity (a.u)

foil, evidencing a metallic Ni state in the formed particles.

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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t (s)

Figure 2. (a) Normalized XANES spectra from Ni K-edge at different reaction times with a comparison of XANES spectra from standard Ni foil and non-crystalline Ni-P nanoparticles; (b) A typical linear combination fitting (LCF) result of the XANES spectrum (120s) with three standard XANES spectra from Ni foil, non-crystalline Ni-P nanoparticles, and the freshly prepared reaction solutions; (c) Compositions of the reaction mixture at different reaction times obtained from the LCF results.


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The possible Ni species included into the reaction mixture at different time were proposed to be Ni2+ hydrates, Ni nanocrystals, and Ni-P nanoparticles, which have similar local structures with Ni-species contained in the freshly prepared reaction solution, Ni foil, and non-crystalline Ni-P nanoparticles, respectively. Therefore, the XANES spectra of the reaction mixture could be considered as a joint contribution from the above three components. As a result, each composition content could be estimated through linear combination fitting (LCF)

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of obtained

XANES spectra (after alignment and normalization) with standard XANES spectra of freshly asprepared reaction solution, Ni foil, and non-crystalline Ni-P nanoparticles. The fitting results at different reaction time were shown in Figure 2 (c) with a typical fitting result shown in Figure 2 (b) (Details in Table S1 and Figure S2). With a linear decrease of Ni2+ hydrates, the content of non-crystalline Ni-P showed a generally linear increase before 180s, except of 30s, probably due to the generation of a small amount of Ni crystals. After 180s, the increase rate of non-crystalline Ni-P decreased gradually with a final content of about 82%. In contrast, the content of Ni2+ hydrates decreased in a similar manner. However, crystalline Ni could not be observed in the fitting results at 300s, indicating a total conversion from Ni2+ hydrates to non-crystalline Ni-P. The k3-weighted Fourier transformation (FT) spectra from Ni K-edge are shown in Figure 3 (a). The outstanding coordination peak (without phase-correction) ranged from 1.6 to 2.0 gradually shifted to higher R side due to the bond-length increase along with the reaction time. Simultaneously, the coordination peak became broader with a de-creasing intensity, indicating the increase of structural dis-order and/or the changes of bonding species around Ni center atoms. Considering about the LCF fitting results, the coordination structure around Ni probably has changed from Ni-O to Ni-Ni, Ni-P, and Ni-O due to the formation of Ni crystal and noncrystalline Ni-P nanoparticles, and the final conversion into non-crystalline Ni-P nanoparticles.


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Figure 3. (a) k3-weighted Fourier transformation spectra of Ni K-edge EXAFS spectra in R space at different reaction times and the standard EXAFS spectra from Ni-P nanoparticles and Ni foil (6 µm); (b) Variations of coordination numbers of Ni-Ni, Ni-O, and Ni-P atom-pairs calculated from the EXAFS fitting results; (c) Bond-lengths of Ni-Ni, Ni-O, and Ni-P atom-pairs obtained from EXAFS fitting results at different reaction times. Therefore, to quantify local structure changes around central Ni atoms, EXAFS fittings were performed using Ni-O, Ni-P, and Ni-Ni atom-pairs extracted from crystalline NiSO4,

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Ni3P,

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and Ni 49 (Detailed fitting results were shown in Table S2 and Figure S3.). As shown in Figure 3 (b) and (c), the initial sample before pulse discharge (0s) observed 6 oxygen atoms coordinated to Ni with a Ni-O band length of 2.06 Å. The absence of Ni-Ni and Ni-P atom pairs at 0s confirmed that Ni species were mainly existed in the form of Ni2+ hydrates. When the reaction time increased to 30 s, Ni-Ni atom-pairs were observed but without Ni-P atom-pairs, verifying the partial reduction of Ni2+ hydrates into Ni atoms with a subsequent formation of Ni clusters or nanocrystals instead of Ni-P structures, which has also been validated by HRTEM observations and LCF results. The absence of Ni-P atom pairs probably implies a slower reduction of P+-


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ligands compared with Ni2+ hydrates. It is noticed that the fitted Ni-Ni coordination number is only 0.5, which is much smaller than the coordination number (12) of bulk Ni. In fact, the fitting coordination number is the apparent coordination number without considering the content of Ni nanocrystals. Taking into account of the con-tent of Ni nanocrystals of 8.1 %, the calculated (or the real) Ni-Ni coordination number could be 6.2. Although this value is still smaller than 12, this might be caused by the size effect 24 of nanocrystals. Supposing the Ni nanocrystals as spherical, then the decrease of coordination number from 12 to 6.2 corresponds to a decrease of particle radius to 1~2 nm, which means that the early-formed Ni nanocrystals have an average diameter of about 3~4 nm, which agrees well with the HRTEM observations. In contrast to the gradual decrease of white line peak from 60 to 210s in Figure 2 (a), the coordination peak in Figure 3 (a) shifted little to larger R side due to the slow formation of Ni-Ni and/or Ni-P atom pairs. Probably, the reduced P atoms have adsorbed on the surface of the formed Ni crystals, preventing further growth of larger nanoparticles. However, after 210s, the coordination peak shifted closer to that of Ni-P nanoparticles, probably due to a rapid formation of Ni-Ni and Ni-P atom pairs shown in Figure 3 (b). Until 300s, the coordination peak position was still smaller than that of Ni-P nanoparticles, this could attribute to containing of Ni-O atom pairs about 12.6 %. Because the bond length of Ni-O is much shorter than both Ni-Ni and Ni-P bonds. However, the bond lengths of Ni-Ni, Ni-O, and Ni-P changed little during the reactions as shown in Figure 3 (c). To explore coordination structure changes around P, P K-edge XAFS spectra were collected at beamline 4B7A in Beijing Synchrotron Radiation Facility (BSRF). However, the spectrum collection from P K-edge was ex situ due to a vacuum atmosphere requirement. Briefly, the reaction solution was dropped on a paper filter at different reaction times, then P K-edge XAFS


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spectra shown in Figure 4 were collected after the solution was dried under ambient conditions. Spectrum from blank paper filter was shown in Figure S4, a nearly linear decreasing along the energy showed little side effects from paper filter to P K-edge XAFS spectra. Figure 4 (a) showed clearly that the peaks corresponding to P 0 and P 5+ (According to standard samples shown in Figure S5) occurred at 60 s, confirming the reduction and oxidation of P 1+ into atomic P and P species with higher valence. Then, the two peaks increased gradually with a decrease of P 1+ peak, indicating a content increase of P 0 and decrease of P 1+. However, the coordination peak of P-Ni in Figure 4 (b) has not been observed at 60 s, instead, the peak has been broadened due to the reduction of P 1+. In contrast, the P-Ni coordination peak was observed at 90 s, confirming the formation of P-Ni atom pairs. After that, the peak intensity of P-Ni coordination increased with the de-crease of P-O peaks due to the further formation of P-Ni contents, which has also been observed in the XANES fit-tings from Ni K-edge spectra. Similar fitting strategies with Ni EXAFS fitting have been used for the fitting of P K-edge spectra, except for the using of P as center atom (Details were shown in the supporting information). The fitting results were shown in Figure 4 (c-d), and Table S3 and Figure S5 in the supporting information. Before the pulse discharge, P was surrounded with about 4 O atoms, which is higher than the theoretical value of 2 due to complicated hydrolysis reactions of Pcontaining species.

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As the reaction continued, the coordination number of O atoms gradually

decreased due to the disproportionation of H2PO2- to P 0 and P species with higher valence. The formation of P 0 resulted into a content decrease of P 1+ containing species, which were mainly sur-rounded by O atoms. On the other hand, the gradual in-crease of P-Ni coordination number was caused by the absorption of the reduced P 0 to the formerly existed Ni crystals. Although the coordination peak of P-Ni has not been observed until 60s, the fitting results showed that a little


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content of P-Ni were contained in the formed particles. That means P atoms have diffused into the already formed Ni crystals without destroying their structures. Thus, this could conclude that the nucleation of Ni-P has started at least from 60s after the reaction was triggered. When the reactions reached to 300 s, the number of coordinated Ni atom was about 7.3, which is smaller than the final non-crystalline Ni-P nanoparticles.

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This decrease could be attributed to partial

existence of phosphates. Considering the LCF XANES fitting results from Ni K-edge, the final content of Ni-P is about 87%, then, 13 % of P-containing species should be in the form of phosphates. As a result, the real coordination number of Ni atom surrounded with P was estimated to be 8.4, which is very close to the calculated 8.2 from P K-edge XAFS spectrum in our previous report. 41


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Figure 4. (a) XANES spectra from P K-edge collected at different reaction times from 0 to 300s with an interval of 30s; (b) k3-weighted Fourier transformation spectra of P K-edge EXAFS correspondingly to the XANES spectra; (c) Variations of coordination-numbers of Ni-Ni, Ni-O, and Ni-P at-om-pairs calculated based on the LCF results from Ni K-edge and EXAFS fitting from P K-edge; (d) Variations of bond-lengths of P-Ni and P-O atom-pairs obtained from the EXAFS fitting from P K-edge. For further study of the particles growth manner after the pulse discharge has triggered the redox reactions, we conducted in situ SAXS experiments at beam line 1W2A also at BSRF. SAXS technique is very efficient for the assessment of nanoparticle size and size distributions,

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therefore, we employed the in-situ synchrotron radiation based SAXS technique to monitor the size evolutions during the particle growth process. The two-dimensional SAXS patterns (shown in Figure S7) were recorded with a Mar165 CCD detector, and then firstly converted into I vs. q curves. After that a model-independent tangent-by-tangent (TBT) method

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was used to get

particle size evolutions from the one-dimensional curve. The experimental SAXS intensities and the simulated ones are compared in Figure 5 (a). It can be seen that the simulated SAXS curves are in good agreements with the experimental ones. The normalized particle volume distributions extracted from the SAXS data are shown in Figure 5 (b).


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Figure 5. (a) Comparison between the experimental SAXS intensities (Symbols) and the simulated ones (solid lines); (b) Normalized particle-volume distribution P(R) extracted from the SAXS data by using the TBT method. From Figure 5, it can be seen that the initial particle sizes have a narrow distribution with an averaged particle diameter about 2 nm at 10s and 3.6 nm at 30s, which are in agreement with the TEM observation as shown in Figure 1. With the increase of the reaction time to 60s, the average particle diameter has increased, but the width of diameter distribution becomes broader. Within the first 60s, the average size of the particles grows up gradually in one unimodal distribution. This result demonstrates that diffusion growth 52 by the transportation of Ni and P atoms onto the Ni nanocrystals is the main growing manner. It is interesting that the volume distribution of particles exhibits a bi-modal feature in a range of reaction time from 90 to 180s. The small-sized one of the double peaks is dominated, and its position change inherits the unimodal feature of the reaction at early stage. For example, the small-sized peak at 120s corresponds to a particle diameter of ~ 5 nm, which is in good agreement with the TEM observation (marked as 120s) as shown in Figure 1. We notice that the large-sized peak position is twofold larger or more than the small-sized peak position. Evidently, the large-sized distribution peak can be attributed to the


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coalescence of the small-sized particles. Indeed, we can observe that partial larger particles have diameter more than twice the diameter of smaller particles as shown in the 120s-HRTEM image. All these SAXS results reveal different growth behaviors of particles in the reaction solution: reaction limited and diffusion limited growths in the period from 90s to 180s. This suggested NiP nanoparticles could grow by monomer attachment from solution and/or by particle coalescence, which has also been confirmed by in situ HRTEM observations

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. When the

reaction proceeds to above 210s, the particle volume distribution recurs to the unimodal distribution, but the most probable particle size is still smaller than the large-sized component just in its foregoing bimodal distribution. This result implies that the small-sized particles, further growing in the reaction limited manner, have a higher growth rate than that of the largesized particles, which mainly growing in the diffusion limited growth manner, especially after 180s. On the one hand, the high-concentration small-sized particles have still a certain coalescence probability to form large-sized particles and a certain collision probability to accept the remaining Ni and P atoms in the reaction solution. On the other hand, the coalesced largesized particles also tend to accept the remaining Ni and P atoms in the solution to repair the particle to have spherical shape for lowering the surface energy of the particles. As a result, the particle volume fraction versus particle diameter returns to a unimodal distribution. Based on all the above all experimental results and analysis, the obscure formation process of non-crystalline Ni-P nanoparticles becomes clear. The formation process includes mainly the following four stages as described by a schematic map shown in Scheme 2.


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Scheme 2. A schematic map of the formation mechanism of non-crystalline Ni-P nanoparticles: redox and nucleation of crystal Ni, nucleation of Ni-P, phase transformation from crystalline to non-crystalline, and growth of non-crystalline Ni-P nanoparticles. Stage I: Nucleation of crystalline Ni. When pulse discharges were released into the reaction solution at a certain temperature. Partial Ni2+ cations in the reaction solution were directly reduced into Ni atoms. When the concentration of Ni atoms reached to the critical saturation level, Ni nuclei were formed. Then the ceaselessly reduced Ni atoms diffused gradually onto the Ni nuclei to form the Ni nanocrystals with FCC structure. These Ni clusters and nanocrystals play a role of catalyst for the subsequent reduction of P anions via the dissociation of H2PO2-. This process lasts about 30s from the beginning. Stage II: Nucleation of Ni-P. Next, the H2PO2- ions were unceasingly reduced into P atoms at the surface of Ni nanocrystals under the catalysis of Ni nanocrystals. The reduced P atoms together with the Ni atoms were attached to the surface of Ni nanocrystals to form Ni-P nuclei. With the progress of the reaction, the ceaselessly reduced Ni and P atoms were continuously adhered to the surface of Ni nanocrystals to form a Ni-P coating layer. Due to the noteworthy difference between Ni and P atomic radii, the coating layer was randomly stacked into a non-


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crystalline structure by Ni and P atoms. In this stage, the diffusion of P atoms from the coating layer to the core of Ni nanocrystals is inevitable. However, the co-deposition of Ni and P atoms on the surface of Ni nanocrystals through the diffusion of Ni in solution and the reduction of P at the surface is the main growth manner of the particles. Roughly, this stage is from 30 to 90s. Stage III: Phase transformation from crystalline to non-crystalline. In this stage, the Ni-P coating layer formed a P-rich region at the surface of Ni nanocrystals, which compelled P atoms to enter into the interior of Ni nanocrystals in a manner of interfacial diffusion, similar phase segregation process has also been monitored using HRTEM reported by Sophie Carenco et al 54. The substitution of smaller P atoms for larger Ni atoms induced a serious lattice distortion in the initial Ni nanocrystals. This diffusion process behaves as an interface movement from outside to inside, leading to the transformation from crystalline Ni to non-crystalline Ni-P. As P atoms diffused into the whole Ni nanocrystals, probably, when the atomic ratio of P increased to more than 20 at. %, 42, 55 the change from crystalline Ni to non-crystalline Ni-P was completed. It was also noticed that both reaction limited growth (small-sized particles) and diffusion growth manner (large-sized particles) have been observed as shown in a bimodal distribution of particle size. However, reaction limited growth manner has dominated the increasing of particle size leading to later a unimodal distribution. The time span of Stage-III is mainly from 90 to 180s. Stage IV: Further growth of non-crystalline Ni-P nanoparticles. Due to the coalescence of particles, the particle concentration in the solution became lower and lower so that the reaction limited growth was almost stopped leaving mainly the diffusion limited growth. However, the remaining Ni and P atoms in the solution were continuously transported onto the non-crystalline Ni-P nanoparticles until the reagents were exhausted. In this stage, the non-crystalline Ni-P particles were gradually repaired to be more spherical for lowering the surface energy. This


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phase can be attributed to a diffusion growth of the non-crystalline Ni-P particles. Duration time started approximately from 180s to the end. CONCLUSIONS The formation process of non-crystalline Ni-P nanoparticles under liquid pulse-discharges was carefully studied by HRTEM observations, in situ XAFS from Ni K-edge, XAFS from P K-edge, and in situ SAXS techniques. The formation process of non-crystalline Ni-P nanoparticles includes mainly four stages, respectively, the nucleation of crystalline Ni, the nucleation of noncrystalline Ni-P, the transformation from crystalline Ni to non-crystalline Ni-P, and the further growth of non-crystalline Ni-P nanoparticles. During the formation process, Ni2+ cations are firstly reduced to form the Ni nuclei leading to the formation of FCC-structured Ni intermediate phase. Then, the reduced P 0 absorpted to the early-formed Ni nanocrystals together with the reduced Ni atoms. The co-deposition of Ni and P atoms on the surface of Ni nanocrystals forms a Ni-P non-crystalline coating layer, and subsequently a Ni-P nucleation. Next, the diffusion of P atoms from outside to inside promotes the transformation of crystalline Ni toward non-crystalline Ni-P. Finally, the non-crystalline Ni-P nanoparticles have a slow growth by the diffusion of the remaining Ni and P atoms onto the particle surface. More interestingly, different growth manners were observed at different reaction times. During the nucleation stages, diffusion limited growth has dominated the growth manner of the growing particles. Then, in the phase structure transformation stage, both reaction limited and diffusion limited growth were observed, but the reaction limited growth manner has a higher growing rate. After the phase structure was transformed into non-crystalline, the diffusion limited growth dominated the particle growth once again.


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * Zhonghua Wu, E-mail: [email protected] * Shujiang Jiao, E-mail: [email protected] * Hongying Yu, E-mail: [email protected]

Funding Sources This work was supported by the fundings coming from China Postdoctoral Science Foundation, the Ministry of Science and Technology of China, and National Natural Science Foundation of China.

ACKNOWLEDGMENT The authors acknowledge financial supports from the Ministry of Science and Technology of China (No. 2017YFA0403000), China Post-doctoral Science Foundation (No. 176947), and the National Natural Science Foundation (Nos. 51374019, 50374010, U1432104, 11405199, 11305198, U1332107) of China.

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TOC Graphic Caption A 4-stage growth process of non-crystalline Ni-P nanoparticles with unexpected phase structure transformation was revealed by in situ XAFS from Ni K-edge, ex situ XAFS from P K-edge, and in situ SAXS.


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In Situ Time Resolved-XAFS and SAXS Revealed an Unexpected

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