Interface Dynamics in Ag-Cu3P Nanoparticle Heterostructures

Abstract

Earth-abundant transition metal phosphides are promising materials for energy-related applications. Specifically, copper(I) phosphide is such a material and shows excellent photocatalytic activity. Currently, there are substantial research efforts to synthesize well-defined metal-semiconductor nanoparticle heterostructures to enhance the photocatalytic performance by an efficient separation of charge carriers. The involved crystal facets and heterointerfaces have a major impact on the efficiency of a heterostructured photocatalyst, which points out the importance of synthesizing potential photocatalysts in a controlled manner and characterizing their structural and morphological properties in detail. In this study, we investigated the interface dynamics occurring around the synthesis of Ag-Cu₃P nanoparticle heterostructures by a chemical reaction between Ag-Cu nanoparticle heterostructures and phosphine in an environmental transmission electron microscope. The major product of the Cu-Cu₃P phase transformation using Ag-Cu nanoparticle heterostructures with a defined interface as a template preserved the initially present Ag{111} facet of the heterointerface. After the complete transformation, corner truncation of the faceted Cu₃P phase led to a physical transformation of the nanoparticle heterostructure. In some cases, the structural rearrangement toward an energetically more favorable heterointerface has been observed and analyzed in detail at the atomic level. The herein-reported results will help better understand dynamic processes in Ag-Cu₃P nanoparticle heterostructures and enable facet-engineered surface and heterointerface design to tailor their physical properties.

Figure 1

(a) HRTEM image of an Ag–Cu nanoparticle heterostructure acquired at 350 °C after H₂ treatment at 500 °C. (b) Corresponding power spectrum with overlaid simulated electron diffraction patterns of the cubic Ag (red) and Cu (green) phases (space group: Fm3m). The power spectra of Ag (red, bottom left) and Cu (green, top right) as insets in (b) highlight the successful tilting of both phases close to their [110] zone axes. Consequently, the heterointerface in (a) is near-parallel to the direction of the electron beam. (c) Ag (Lα₁) and (d) Cu (Kα₁) STEM-EDS elemental maps of the Ag–Cu nanoparticle heterostructure shown in (a).

Figure 2

Selected averaged frames of a HRTEM movie (see Movie S1) at (a) 23.350 s, (b) 24.750 s, (c) 36.750 s, and (e) 51.500 s after the supply of PH₃ to the Ag–Cu nanoparticle heterostructure shown in Figure 1, which was kept at 350 °C. (a) Cu₃P phase nucleates at the triple-phase boundary indicated by a white arrow and (b) grows by consuming the Cu phase. (c) Phase transformation is far advanced, and the different phases, the heterointerfaces I1–3, and the Cu₃P facets F1–3 are highlighted. (d) Power spectrum corresponding to (c) with overlaid simulated electron diffraction patterns of the different phases^, shows their crystallographic relation, and the black ellipse indicates the Ag(111)/Cu(111) heterointerface I1, which remains unaltered when compared to the power spectrum in Figure 1b. The Cu₃P phase (blue) is oriented close to its [0001] zone axis. (e) Strong fringes at the heterointerface I2 caused by strain effects are visible just before the complete transformation of the Cu phase. The white arrow highlights the wetting of the Cu₃P facet F1 by Ag as part of the structural rearrangement. (f) Simulated reflections in the black ellipse match with the power spectrum corresponding to (e) and show an anticlockwise in-plane rotation of the Ag phase relative to the Cu and Cu₃P phases when compared with those in (d). The Cu(111) planes show an in-plane angular relation with Cu₃P(3030) planes during the rearrangement, which is indicated by the black arrow in (f) and also observable in (d).

Figure 3

(a) Selected averaged frames of the same HRTEM movie used in Figure 2 (see Movie S1). The reaction between Cu and PH₃ is nearly completed 53.700 s after the supply of PH₃, and a white arrow indicates the location of the Cu phase. The power spectrum in (b) corresponds to the white rectangle in (a), and the overlaid simulated electron diffraction patterns of the three involved phases show that the in-plane angular relation between the Cu(111) and Cu₃P(3030) planes is slightly altered (black arrow). Instead, the Ag, Cu, and Cu₃P reflections highlighted via black ellipses in (b) reveal a well-matching in-plane angular alignment. The power spectra of Cu (green, bottom left) and Cu₃P (blue, top right) are shown as insets in (b).

Figure 4

(a) Selected averaged frames of an HRTEM movie (see Movie S1) representing the state of the Ag–Cu₃P nanoparticle heterostructure 83.200 s after the supply of PH₃. The phase transformation is completed, and two heterointerfaces, I4 and I5, are visible. Moreover, fringes are located in the Ag phase close to heterointerface I4, which indicates strain effects. (b) Power spectrum corresponding to (a) with overlaid simulated Ag and Cu₃P electron diffraction patterns. The insets in (b) represent power spectra from the Ag (red, bottom left) and the Cu₃P (blue, top right) phases. (c) Zoomed-in section of the power spectrum (brightness and contrast adapted) indicated by a black rectangle in (b) shows the Ag(220) reflections. Three slightly different Ag(220) reflections labeled as regions R1–3 are highlighted by red arrows. (d–f) Inversed power spectra after applying a spot mask on (d) R1, (e) R2, and (f) R3 and their associated Ag(220) reflections in the power spectrum in (b). The reflections in (c) can be assigned to different regions within the Ag phase, highlighted in (a).

Figure 5

(a) HRTEM image of the same Ag–Cu₃P nanoparticle heterostructure acquired 200 min after the start of the PH₃ supply. The temperature and the PH₃ flow remained unaltered during this period, and heterointerface I5 grew at the cost of heterointerface I4. (b) Strain map of the Ag–Cu₃P nanoparticle heterostructure obtained from GPA of the phase image of the reconstructed exit wave function. The spot masks for the GPA were centered at the Ag(111) and Ag(111) reflections and the map visualizes strain in the Ag[220]/Cu₃P[3360] direction. The labels highlight the observed regions in (d–f). (c) Power spectrum corresponding to (a) with overlaid simulated electron diffraction patterns of the Ag and Cu₃P phases allows for the assignment of well-aligned Ag(220) and Cu₃P(3360) planes forming heterointerface I5 (black ellipse). The matching Ag (red) and Cu₃P (blue) reflections in the black circle indicate that the planes perpendicular to heterointerface I5 have the same interplanar spacing in both phases, which could hint toward the origin of the rearrangement to a more stable configuration. The power spectra of the Ag (red, bottom left) and Cu₃P (blue, top right) phases are shown as insets of (c). (d) Phase image of heterointerface I5 with simulated phase images of the Ag (bottom left) and Cu₃P (top right) phases as insets. (e) Phase image of the stepped heterointerface I4. Red (Ag) and blue (Cu₃P) lines indicate the presence of dislocations due to lattice misfit. Ag(111) and Cu₃P(0330) planes straight at the heterointerface have a parallel in-plane alignment while showing an in-plane rotation further away from the heterointerface. (f) Phase image with fringes (indicated by white arrows) due to strain effects evolving in the Ag phase parallel to heterointerface I5.

Figure 6

(a) HRTEM image of the same Ag–Cu₃P nanoparticle heterostructure after increasing the temperature to 500 °C shows the complete replacement of heterointerface I4 by heterointerface I5. (b) Power spectrum corresponding to (a) with overlaid simulated electron diffraction patterns of the Ag and Cu₃P phases. The thermal expansion of the Ag phase upon heating the sample to 500 °C has been considered for the simulation. The insets in (b) reveal selected area power spectra of the Ag (red, bottom left) and Cu₃P (blue, top right) phases.