Chapter 4. Nanoarchitecture of epitaxially grown copper phosphide (Cu 3 P) nanosheets for energy
4.1. Background and objective
Recently, transition metal phosphides (TMPs) have been studied for energy applications as anodes for lithium ion batteries (LIBs) and as electrocatalysts in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)32, 141-142. TMP is considered one of the potential candidates for anode materials in LIB due to its higher gravimetric and volumetric capacities than graphite, which is currently the most widely used as LIB anode15-17. In HER and OER, TMP is increasingly being used as an acid- stable and active catalyst to replace rare and expensive noble metals18-20. Even so, TMP’s performance in practical energy applications currently remains lower than expected. This is mainly due to the problem of contact between the TMP active material and the metal current collector of the anode. A conductive binder adhesive is used to attach the TMP to the metal which often leads to capacity loss by inhibiting ion transport from the electrolyte143-144. To avoid this problem, direct growth of TMPs on current collectors has been extensively investigated. Another problem that has a significant impact on performance is that the TMP based LIB anode undergoes significant volume change during the charge and discharge cycles, resulting in TMP separation or removal from the current collector145. To apply TMP to HER, a high surface area to maximize contact with the electrolyte is required for high electrocatalytic efficiency146-147. As a result, various forms of TMP such as nanosheet, nanopillars, nanoparticle, and porous architectures of TMPs provide both increased buffer space to accommodate volume changes and high surface area for optimal electrocatalytic efficiency15, 42, 141, 148. However, manufacturing these complex architecture on current collectors involves the tedious additional processes of patterning, etching or growing an intermediate such as CuOH15, 19, 42. Therefore, the direct manufacture of TMP nanoarchitecture on the current collector is highly desirable.
Here, we report the direct and epitaxial growth of copper phosphide (Cu3P) nanosheets, one of TMPs, on copper (Cu) foil by solid-vapor reaction. The thickness of the Cu3P nanosheet varied in the range of 30 nm to 400 μm by controlling the chamber pressure during growth. Interestingly, the as-grown Cu3P nanosheets were found to be vertically aligned on the Cu foil. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) results revealed that this alignment is due to epitaxial relations between Cu3P and Cu. The epitaxial growth and nanosheet thickness control show that nanoarchitecture of Cu3P nanosheets can be fabricated directly on Cu foil in a single step without any additional treatment. The Cu3P nanoarchitecture was tested as an anode for LIB and HER compared to the film morphology of Cu3P. The Cu3P anode with nanoarchitecture showed higher capacity and rate capability in LIB compared to Cu3P with a film morphology. However, in the case of HER, the electrocatalytic efficiency
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of the nanoarchitecture was high only in the first cycle, and then the performance deteriorated due to the morphological reconstruction of Cu3P during operation.
4.2. Experimental methods
Fabrication of copper phosphide (Cu3P) with nanostructured architecture on Cu foil.
Based on a reported method149, Cu foil (99.8%, Alfa Aesar) was annealed to make a large domain sized crystal plane with predominantly the (200) plane on the surface. The annealed Cu foil was placed in a tube furnace with 0.2 g red phosphorus (> 97 %, Sigma-Aldrich). The Cu foil was 15 cm from the red phosphorus. The tube furnace was heated at 300 ºC for 2 hours to form Cu3P. Growth was carried out at different chamber pressures. A schematic diagram of the process is shown in Figure 4.1.
Figure 4.1. Schematic diagram of manufacturing Cu3P nanoarchitecture on Cu foil.
Characterization.
Scanning electron microscopy (SEM) images were taken with Hitachi S-4800 microscope and X-ray photoelectron spectroscopy (XPS) was performed with Thermo Fisher K-alpha system. X-ray diffraction (XRD) patterns were obtained using Rigaku D/Max2500V with 1.5406 Å Cu Kα radiation.
Transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and selected area electron diffraction (SAED) were carried out with JEOL JEM-2100F equipped with a Cs probe corrector at the accelerating voltage of 200 kV. Cross- sectioned TEM samples were prepared using a focused ion beam (FIB) instrument (FEI Quanta 3D FEG) equipped with an electron backscattered diffraction (EBSD) detector that allowed to determine grain sizes and crystallographic planes of the Cu foil substrates.
Electrochemical Measurements.
To investigate the rate capability of Cu3P-5 and Cu3P-300, we prepared half cells (CR2032) as follows.
The cells were assembled using synthesized samples as a working electrode, polypropylene as a separator, lithium foil as the counter electrode, and 1 M LiPF6 in ethylene carbonate (EC)/ethyl methyl
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carbonate (EMC) (3:7 v/v) as the electrolyte (provided by PanaxEtec). The charge-discharge experiments were performed galvanostatically over a voltage range of 0.02 - 3 V vs. Li+/Li with different currents at room temperature. (1C = 363 mAh g-1). Electrocatalytic measurements were carried out using a 3-electrode cell and a 0.5 M sulfuric acid (H2SO4) electrolyte solution. Graphite rod (Sigma Aldrich) as a counter electrode and Ag/AgCl reference electrode were used. Grown Cu3P samples were directly used to measure electrocatalytic measurements. The linear sweep voltammetry was performed with 10 mV/s scan rate using a potentiostat (Zive SP2, ZIVE LAB).
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4.3. Nanoarchitecture of Cu
3P nanosheets on copper foil
Cu3P was grown by the reaction of red phosphorus with the annealed Cu foil. During the heat treatment, red phosphorus vaporized and reacted with the surface of the Cu foil to form Cu3P. Cu3P growth was tested at various chamber pressures (5, 170, 220, and 300 mTorr) to investigate effect of pressure on Cu3P growth (Figure 4.2). Figure 4.2a-c show images of Cu3P sheets grown at various pressures of 5, 170 and 300 mTorr. The thickness of Cu3P sheets depends on the chamber pressure.
With increasing pressure, the thickness increased from ~28 nm to ~440 μm. At 5 mTorr pressure, a nanoarchitecture is formed (Cu3P-5 in Figure 4.2a), but the morphology is more film-like at 300 mTorr (Cu3P-300 in Figure 4.2c). However, the Cu3P sheet morphology is consistently maintained regardless of thickness. The formation of nanosheets is thought to be due to the different growth rates of different Cu3P facets. To quantify this effect, we calculated the density of dangling bonds (which is related to the surface stability for a crystalline facet150) based on a previously reported bonding configuration of Cu3P151 (Table 4.1 and Figure 4.3). In the examined bonding configuration, the density of dangling bonds (DBs) for the (001) and (100) planes are 32.41 and 51.59 /nm2, respectively. The lower DB density of the (001) plane means that this plane has higher surface stability than the (100) plane, indicating that growth perpendicular to (001) is slower than that perpendicular to (100). This difference in growth rate between the different planes results in sheet morphology of Cu3P crystals with larger (001) facets.
EDS, XPS and HAADF-STEM were performed to study the crystal structure of the Cu3P nanosheets in detail. The EDS results confirmed that only Cu and P were present and no other impurity elements were present (Figure 4.4c). The XPS of Cu3P-5 shows two peaks of phosphorus with binding energies of 129.5 and 133.18 eV (Figure 4.4a). The 133.18 eV peak indicates the presence of phosphate (POx) that may form on the surface of Cu3P. Another peak (129.5 eV), which has a lower binding energy than elemental phosphorus (130.2 eV), is due to phosphorous bonded to copper. These attributions are consistent with reported XPS studies on Cu3P152-153. Atomic resolution HAADF-STEM image (Figure 4.4b) shows that the grown Cu3P has no structural defects and the structure is consistent with the atomic model of Cu3P (001). The combined results of EDS, XPS and HAADF-STEM confirm the high crystallinity of the grown Cu3P nanosheets.
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Figure 4.2. Growth of Cu3P on an annealed Cu foil. (a-c) SEM images of Cu3P architectures under (a) 5 (Cu3P-5), (b) 170 (Cu3P-170) and (c) 300 mTorr (Cu3P-300) chamber pressure. Scale bars are 5 μm. (d) Change in thickness of Cu3P nanosheet with chamber pressure.
(001) plane (100) plane
Plane Area 0.432 nm2 0.504 nm2
The number of dangling bonds 14 26
Density of dangling bonds 32.41 /nm2 51. 59 / nm2
Table 4.1. The number and density of dangling bonds (DB) in (001) and (100) planes of Cu3P.
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Figure 4.3. The number of bonding in (001) and (100) plane of Cu3P, which is used to calculate the density of dangling bonds (DB) in Table 4.1.
Figure 4.4. Evaluation of crystallinity characteristics of Cu3P. (a) XPS of phosphorus binding energies for Cu3P-5. (b) HAADF-STEM image of Cu3P nanosheets overlaid atomic model of Cu3P (001). Scale bar is 1 nm. (c) EDS spectrum of Cu3P-5 nanoarchitecture.
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4.4. Epitaxial relation between Cu
3P nanosheet and surface of Cu foil
Interestingly, it was observed that the Cu3P sheets on Cu foil were evenly aligned in the same direction in case of Cu3P-5 (Figure 4.2a). To further verify the origin of this alignment, we examined the alignment in a Cu3P sample in the early stage of growth. Figure 4.5 shows Cu3P prepared with a much smaller amount of red phosphorus (0.05 g) showing a similar alignment. These observations strongly indicate that Cu3P grows epitaxially on the Cu foil, further confirmed epitaxy from XRD.
Figure 4.6a shows XRD patterns of Cu3P-5 and Cu3P-300 on Cu foil. Only two strong peaks at the 2θ values of 44.9 ° and 50.359 °, corresponding to Cu3P (300) and Cu (200) reflections (JCPDS card Nos 03-065-3628 and 01-071-0610, respectively). The surface of Cu foil before and after annealing was analyzed by electron backscatter diffraction (EBSD) (Figure 4.6b). Upon annealing,the Cu grain size, which was originally on the order of tens of micrometers in random orientation, grew to hundreds of micrometers with predominantly the Cu (200) plane. XRD and EBSD results confirm that the Cu3P sheets are epitaxially grown on Cu (200) surface.
By examining the back side of the Cu foil where Cu3P nanosheets are more sparsely distributed than the front side (since only phosphorus vapor that permeated between the quartz substrate and Cu foil was available for the reaction), we further investigated the morphology and alignment of the Cu3P nanoarchitecture (Figure 4.7a). It can be seen that Cu3P nanosheets have a hexagonal truncated morphology and two distinctive features: 1) Cu3P nanosheets always grow along two orthogonal directions on the Cu foil, 2) the vertices of the truncated hexagonal Cu3P nanosheets point in the same direction. Next, the interface between the Cu3P nanosheet (Cu3P-5) and the Cu foil was analyzed by TEM (Figure 4.7b-d). Figure 4.7b is a low magnification cross-sectional TEM image of Cu3P nanosheets. The thicknesses of the two nanosheets are in the same range as measured by SEM. The atomic-resolution cross-section TEM image clearly shows that a Cu (100) plane forms the interface between the Cu3P nanosheets and the Cu surface (shown by the yellow line in Figure 4.7c) This is consistent with our EBSD result. In addition, the Cu3P (001) plane is aligned parallel to the Cu (022) plane. When the interface was analyzed along the <001> zone axis of Cu3P as shown in Fig. 3d, the Cu (100) interface was also observed here, pointing that Cu/Cu3P interface is always the Cu (100) plane.
At the Cu3P <001> zone axis, the Cu3P (210) plane is aligned to the Cu (111) plane with a 6.6 % lattice mismatch. These interfaces show the existence of two epitaxial relationships: 1) Cu3P (001)/Cu (022) and 2) Cu3P (210)/Cu (111). These are clearly shown in the corresponding atomic model in Figure 4.7e.
The two epitaxial relationships explain the two characteristic morphological features of the Cu3P nanosheets nanoarchitecture described in Figure 4.7a. First, due to the Cu3P (001)/Cu (022) epitaxial relationship, two orthogonal Cu3P nanosheets derived from the Cu {022} family of planes on the Cu (100) surface (Figure 4.7a and 4.8). Secondly, Cu3P (210)/Cu (111) epitaxy results in all the vertices of the truncated hexagonal Cu3P nanosheets with the same direction, as shown in Figure 4.7a. Due to the two epitaxial relationships, truncated hexagonal Cu3P nanosheets with the same vertex direction
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preferentially grow along two orthogonal directions on the Cu foil, resulting in the observed nanoarchitecture. Moreover, these two epitaxial relationships differ from the previously reported relationship between Cu and Cu3P154. The reason is due to the different types of interfaces. In this case, Cu (100) is formed as the interface between Cu and Cu3P, but Cu (133̅) and Cu (221) are formed as the interface in the previous report. Therefore, it shows that Cu surface engineering is required to produce the nanoarchitecture of aligned Cu3P nanosheets.
Figure 4.5. Initial growth of Cu3P. SEM image of Cu3P architecture produced with reduced amount of red phosphorus (0.05 g) at 5 mTorr. Scale bar is 1 μm.
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Figure 4.6. Crystal orientation of Cu3P and surface of Cu foil. (a) XRD patterns of Cu3P architectures on Cu; Cu3P-5(black) and Cu3P-300 (red). (b) SEM images and EBSD mappings of the Cu foil surface before and after annealing at 1000 ºC. Scale bars are 300 μm.
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Figure 4.7. Epitaxial relation between Cu3P nanosheet and surface of Cu foil. (a) SEM image of sparsely grown Cu3P nanosheets on the backside of the Cu foil. Scale bar is 10 μm. (b) Cross-sectional TEM image of Cu3P nanosheets. Scale bar is 20 nm. (c,d) Atomic resolution TEM images of the interface between Cu3P and Cu at the Cu3P (c) <100> and (d) <001> zone axes. Bottom right images are digital diffractograms of the TEM images. Scale bars are 1 nm. (e) Atomic model corresponding to Figure 4.6c and d.
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Figure 4.8. Atomic model of the Cu (100) surface. There are two {022} family planes orthogonal to each other on Cu (100) surface.
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4.5. Comparison of Cu
3P nanoarchitecture with film architecture as energy applications
Finally, the electrochemical properties of Cu3P nanoarchitecture were studied by using it as anode in a LIB and as electrocatalyst for the HER. By comparing the performance of the nanoarchitectural form (Cu3P-5) with that of the film-like morphology of Cu3P (Cu3P-300), the nanoarchitectural effect was clarified.
Figure 4.9a shows the charge and discharge potential curves for Cu3P nanosheets nanoarchitecture (Cu3P-5). The discharge curve has two obvious plateaus near 0.89 and 0.8 V, which are associated with the formation of LixCu3-xP and complete conversion into Li3P. The charging curve has three plateaus ~ 0.72, 1.09, and 1.26 V related to the slow loss of lithium, conversion of Li3P into Li3-xCuxP, and recovery of Cu3P155-156. Figure 4.9b compares the discharge rate capabilities of Cu3P-5 (red) and Cu3P-300 (blue).
In the former case, the discharge increased from 298.4 mAh/g to 327.6 mAh/g in the first 3 cycles at 0.05 C, but slowly stabilized during the next cycles. Retesting the rate at 0.2 C after 21 cycles restored the initial capacity to a slightly higher value than the previous 0.2 C test. In the case of Cu3P-300 in film-like morphology, the overall capacity was lower than for the nanoarchitecture. Moreover, as the number of cycles increased, the capacity decreased further in contrast to the nanoarchitecture. An optical image showing half of a coin cell batteries of the two Cu3P samples after rate capability test is shown in Figure 4.10. The Cu3P nanoarchitecture (Cu3P-5) maintained its initial state after the test, but the film-like morphology (Cu3P-300) was peeled off from the Cu foil. The stability of the nanoarchitecture is believed to be due to the presence of sufficient buffer space to accommodate the volumetric expansion that occurs during charging and discharging. Film-like morphology cannot accommodate this expansion due to its compact architecture. These results show that nanoarchitecture has clear advantages over the film-like morphology when using Cu3P nanosheets as a LIB anode.
Figure 4.9. Half-cell battery performance of nanoarchitecture and film architecture of Cu3P. (a) Charge and discharge voltage profiles of Cu3P-5 nanoarchitecture at different rates and (b) rate capability of Cu3P-5 (red) and Cu3P-300 (blue).
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Figure 4.10. Comparison of architectural collapse after cycling. Photograph of Cu3P/Cu produced at 5 mTorr (left) and 300 mTorr (right) after electrochemical rate capability tests.
We tested the electrocatalytic activity of the nanoarchitecture of Cu3P nanosheets for the HER. Figure 4.11a shows HER polarization curves of Cu3P with nanoarchitecture and film morphologies. The measurement was performed in 0.5 M H2SO4 with a scan speed 10 mV/s. In the first cycle, Cu3P nanoarchitecture showed a lower overpotential (291mV at 10mA/cm2) and higher electrocatalytic activity compared to the film- like Cu3P (551mV at 10 mA/cm2). However, the HER efficiency of the nanoarchitecture decreased slightly as the number of cycles increased. In contrast, the performance of the film-like Cu3P increased with the number of cycles, similar to that of nanoarchitecture at the end of 1000 cycles. This behavior is also observed in the Tafel slopes (Figure 4.11b), where Tafel slope of the Cu3P nanoarchitecture changed slightly from 83.7 to 83.1 mV/dec, while that of the film-like Cu3P decreased significantly from 126.9 to 72.6 mV/dec. To illustrate this behavior, we used SEM to study the morphology of Cu3P electrodes after 1000 cycles of HER. The nanoarchitecture of Cu3P nanosheets changed significantly after 1000 cycles, and the nanoarchitecture was reconstructed to form microparticles (Figure 4.11c). This morphology reconstruction is clearly responsible for the reduced HER efficiency. Interestingly, similar microparticles were observed in Cu3P with film-like morphology after 1000 cycles of HER (Figure 4.11d). This suggests that morphology of Cu3P is reconstructed and forms microparticles during HER cycling in H2SO4 solution regardless of the initial morphology.
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Figure 4.11. HER and morphology reconstruction. (a) HER polarization curves and (b) Tafel plots of Cu3P-5 (red) and Cu3P-300 (black) pressure for 1 (solid lines) and 1000 (dash lines) cycles. (c,d) SEM images of Cu3P architectures (c) Cu3P-5 and (d) Cu3P-300 after 1000 cycles of HER.
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4.6. Summary
In summary, this study shows that nanoarchitecture of Cu3P nanosheets can be directly manufactured by controlling the thickness of Cu3P nanosheets and performing substrate engineering to ensure epitaxial relations between Cu3P and the Cu foil. The thickness of Cu3P sheets was controlled from tens of nanometers to hundreds of micrometers by varying the chamber pressure during the growth. In addition, Additionally, the investigated epitaxial relations, Cu3P (001)/Cu (002) and Cu3P (210)/Cu (111), affected the alignment of Cu3P nanosheets. By directly manufacturing nanoarchitecture of Cu3P nanosheets on Cu foil, we were able to evaluate the performance of a LIB anode. The Cu3P nanoarchitecture showed higher capacity and durability compared to Cu3P with film-like morphology.
In case of HER, the electrocatalytic efficiency of the nanoarchitecture was excellent at the initial cycle, but both forms showed the same performance at the end of 1000 cycles. This behavior is believed to be due to the reconstruction of Cu3P morphology during cycling. Our work provides an easy way to manufacture TMP nanoarchitectures directly on current collectors and highlights the impact of nanoarchitecture on the performance of TMPs in typical energy applications.
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Chapter 5. Summary
In this researches, I studied phosphorus-based materials, especially for 2D black phosphorus (BP) and copper phosphide (Cu3P), at atomic-scale using transmission electron microscopy (TEM).
Firstly, intercalation method was used to modulate property of BP and the spatially controlled intercalation was achieved within angstrom-scale. We discovered that Cu atoms are intercalated in a periodic manner along zigzag direction and confined in angstrom-channels. In addition, it was also identified how the atomic structure of Cu intercalated BP influences morphology (i.e., macroscopic deformation) of the BP crystal. For the study of intercalation mechanism, in situ TEM/STEM imaging unraveled the unraveled the unusual intercalation mechanism according to which Cu atoms intercalate along the top-down direction of BP, this process being rate-limiting. Top-down (c-axis) intercalation is unusual considering reported intercalation mechanisms in 2D materials where the a- or b-axis direction is considered to be the dominant intercalation direction. The electronic properties of BP were also found to be profoundly influenced by Cu intercalation. Cu intercalation significantly changes electronic band structure of pristine BP resulting in semimetallic property, different from the semiconducting nature of pristine BP. We succeeded in measuring local variations in electronic conductivity at Cu intercalated regions and found semimetallic behavior. Our results demonstrate that intercalation can be successfully used to modulate properties of anisotropic 2D single crystal BP at angstrom scale. Moreover, we suggest that this comprehensive and systematic study including the intercalation process, microstructural changes, mechanism, and local property measurements at atomic-scale is a new advancement in intercalation of 2D materials.
Secondly, stability and dynamics of surface nanobubble were investigated with black phosphorus liquid cell (BPLC). Here, using favorable oxygen absorption ability of black phosphorus, we naturally formed gas-filled surface nanobubbles in the BPLC. Due to its high stability even under electron beam, dynamics of surface nanobubble can be directly investigated using TEM in enough spatial and time resolution. The main results and conclusions of the investigated dynamics and stability of gas-filled surface nanobubble are summarized as below
1. It was found that the shrinkage of surface nanobubble is governed by dissolution kinetics, which experimental result directly agrees with a theoretically described equation.
2. We, as a first time, observed pinch-off dynamics of surface nanobubble, which shows a totally different behavior with previously reported pinch-off dynamics of microbubbles.
3. There were abnormal behaviors related with surface pinning effect of surface nanobubble in terminal phases of shrinkage and pinch-off dynamics, indicating that stability of surface nanobubble is related with the surface pinning effect.
Thus, this study not only demonstrates dynamics and stability of surface nanobubble, but also suggests