Redox Chemistries for Vacancy Modulation in Plasmonic Copper Phosphide Nanocrystals

Copper phosphide (Cu3-xP) nanocrystals are promising materials for nanoplasmonics due to their substoichiometric composition, enabling the generation and stabilization of excess delocalized holes and leading to localized surface plasmon resonance (LSPR) absorption in the near-IR. We present three Cu-coupled redox chemistries that allow postsynthetic modulation of the delocalized hole concentrations and corresponding LSPR absorption in colloidal Cu3-xP nanocrystals. Changes in the structural, optical, and compositional properties are evaluated by powder X-ray diffraction, electronic absorption spectroscopy, 31P magic-angle spinning solid-state nuclear magnetic resonance spectroscopy, and elemental analysis. The redox chemistries presented herein can be used to access nanocrystals with LSPR energies of 660-890 meV, a larger range than has been possible through synthetic tuning alone. In addition to utilizing previously reported redox chemistries used for copper chalcogenide nanocrystals, we show that the largest structural and LSPR modulation is achieved using a divalent metal halide and trioctylphosphine. Specifically, nanocrystals treated with zinc iodide and trioctylphosphine have the smallest unit-cell volume (295.2 Å3) reported for P63cm Cu3-xP, indicating more Cu vacancies than have been previously observed. Overall, these redox chemistries present valuable insight into controlling the optical and structural properties of Cu3-xP.

Cage-Confinement Pyrolysis Strategy to Synthesize Hollow Carbon Nanocage-Coated Copper Phosphide for Stable and High-Capacity Potassium-Ion Storage

Metal phosphides with a high theoretical capacity and low redox potential have been proposed as promising anodes for potassium-ion batteries (PIBs). A reasonable configuration design and introduction of a hollow structure with adequate internal void spaces are effective strategies to overcome the volume expansion of metal phosphides in potassium-ion batteries. Herein, we report a cage-confinement pyrolysis strategy to obtain hollow nanocage-structured nitrogen/phosphorus dual-doped carbon-coated copper phosphide (Cu3P/CuP2@NPC), which exhibits a high initial charge capacity (409 mA h g-1 at 100 mA g-1) and an outstanding cycle performance (100 mA h g-1 after 5000 cycles at 1000 mA g-1) as an anode material for PIBs. The novel hollow nanocage structure could prevent volume expansion during cycling and reduce the electron/ion diffusion distance. Besides, the nitrogen/phosphorus dual-doped carbon-coated layer could promote electronic conductivity. In situ X-ray diffraction (XRD) measurements are conducted to study the potassiation/depotassiation mechanism of Cu3P/CuP2@NPC and reveal the structure stability during the cycle process, which further proves that the design ideas of the conductive carbon layer and the hollow structure with adequate internal void spaces are successful.

Copper Phosphide Nanoparticles Used for Combined Photothermal and Photodynamic Tumor Therapy

Copper-based nanomaterials are widely used in near-infrared (NIR) light-mediated deep tumor treatment because of their abundant photothermal and photodynamic properties. However, copper phosphide (Cu₃P) nanoparticles (NPs) are rarely investigated. Herein, Cu₃P NPs were prepared to strengthen their local surface plasmon resonance absorption in the NIR region, exhibiting promising photothermal and photodynamic properties. After surface modification by polyethylene glycol, the formed pCu₃P NPs showed negligible influence on the viability of 4T1 cells, presenting remarkable biocompatibility. However, with 808 nm irradiation, pCu₃P NPs could induce HSP70 and HO-1 protein expression and enhance intracellular reactive oxygen species levels, leading to dramatic cell death. In 4T1 tumor-bearing mice, an intravenous injection of biocompatible pCu₃P NP could lead to remarkable aggregation in the tumor region and significantly inhibit tumor growth under 808 nm laser irradiation, presenting great potential for tumor therapy.

Electrocatalytic CO2 Reduction over Cu3P Nanoparticles Generated via a Molecular Precursor Route

The design of nanoparticles (NPs) with tailored morphologies and finely tuned electronic and physical properties has become a key strategy for controlling selectivity and improving conversion efficiency in a variety of important electrocatalytic transformations. Transition metal phosphide NPs, in particular, have emerged as a versatile class of catalytic materials due to their multifunctional active sites and composition- and phase-dependent properties. Access to targeted transition metal phosphide NPs with controlled features is necessary to tune the catalytic activity. To this end, we have established a solution-synthesis route utilizing a molecular precursor containing M-P bonds to generate solid metal phosphide NPs with controlled stoichiometry and morphology. We expand here the application of molecular precursors in metal phosphide NP synthesis to include the preparation of phase-pure Cu3P NPs from the thermal decomposition of [Cu(H)(PPh3)]6. The mechanism of [Cu(H)(PPh3)]6 decomposition and subsequent formation of Cu3P was investigated through modification of the reaction parameters. Identification and optimization of the critical reaction parameters (i.e., time, temperature, and oleylamine concentration) enabled the synthesis of phase-pure 9-11 nm Cu3P NPs. To probe the multifunctionality of this materials system, Cu3P NPs were investigated as an electrocatalyst for CO2 reduction. At low overpotential (-0.30 V versus RHE) in 0.1 M KHCO3 electrolyte, Cu3P-modified carbon paper electrodes produced formate (HCOO-) at a maximum Faradaic efficiency of 8%.

Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu3-xP Nanocrystals

N-heterocyclic carbenes (NHCs) are an important class of ligands capable of making strong carbon-metal bonds. Recently, there has been a growing interest in the study of carbene-ligated nanocrystals, primarily coinage metal nanocrystals, which have found application as catalysts for numerous reactions. The general ability of NHC ligands to positively affect the catalytic properties of other types of nanocrystal catalysts remains unknown. Herein, we present the first carbene-stabilized Cu_3-xP nanocrystals. Inquiries into the mechanism of formation of NHC-ligated Cu_3-xP nanocrystals suggest that crystalline Cu_3-xP forms directly as a result of a high-temperature metathesis reaction between a tris(trimethylsilyl)phosphine precursor and an NHC-CuBr precursor, the latter of which behaves as a source of both the carbene ligand and Cu⁺. To study the effect of the NHC surface ligands on the catalytic performance, we tested the electrocatalytic hydrogen evolving ability of the NHC-ligated Cu_3-xP nanocrystals and found that they possess superior activity to analogous oleylamine-ligated Cu_3-xP nanocrystals. Density functional theory calculations suggest that the NHC ligands minimize unfavorable electrostatic interactions between the copper phosphide surface and H⁺ during the first step of the hydrogen evolution reaction, which contributes to the superior performance of NHC-ligated Cu_3-xP catalysts as compared to oleylamine-ligated Cu_3-xP catalysts.

Phosphorus-Rich CuP2 Embedded in Carbon Matrix as a High-Performance Anode for Lithium-Ion Batteries

Phosphorus-rich CuP₂ and its carbon composites have been investigated as an anode material for lithium-ion batteries. Through a facile, low-cost mechanochemical reaction, microsized composites composed of active CuP₂ particles uniformly embedded in the carbon matrix have been successfully synthesized. Combined structural and electrochemical characterizations show that phosphorus-rich CuP₂ undergoes irreversible reaction with lithium, giving metal-rich Cu₃P and amorphous phosphorus at the end of the first cycle. Both Cu₃P and phosphorus are reversibly formed in subsequent cycles, contributing to a high reversible capacity of >1000 mA h g^-1. By controlling the carbon content, the electrochemical reversibility and stability of CuP₂ are greatly improved. The carbon composite demonstrates a remarkable lithium-storage capability in terms of a stable capacity of >720 mA h g^-1 over 100 cycles at 200 mA g^-1, a high initial Coulombic efficiency of ∼83%, and a good rate capability with a capacity of >637 mA h g^-1 at 1.6 A g^-1. The performance improvement is mainly associated with the formation of the conductive carbon network that offers high conductivity and fast reaction kinetics, as well as enhanced structural stability of CuP₂ anode.

Self-Supported Cedarlike Semimetallic Cu3P Nanoarrays as a 3D High-Performance Janus Electrode for Both Oxygen and Hydrogen Evolution under Basic Conditions

There has been strong and growing interest in the development of cost-effective and highly active oxygen evolution reaction (OER) electrocatalysts for alternative fuels utilization and conversion devices. We report herein that semimetallic Cu3P nanoarrays directly grown on 3D copper foam (CF) substrate can function as effective electrocatalysts for water oxidation. Specifically, the surface oxidation-activated Cu3P only required a relatively low overpotential of 412 mV to achieve a current density of 50 mA cm(-2) and displayed a small Tafel slope of 63 mV dec(-1) in 0.1 M KOH solution, on account of the collaborative effect of large roughness factor (RF) and semimetallic character. Following that, investigations into the mechanism revealed the formation of a unique active phase during the water oxidation process in which conductive Cu3P was the core covered with a thin copper oxide/hydroxide layer. Moreover, this Cu3P 3D electrode was also applied to the hydrogen evolution reaction (HER) and showed good catalytic performance and stability under the same basic conditions.