Surveying the Synthesis, Optical Properties and Photocatalytic Activity of Cu3N Nanomaterials

This review addresses the most recent advances in the synthesis approaches, fundamental properties and photocatalytic activity of Cu₃N nanostructures. Herein, the effect of synthesis conditions, such as solvent, temperature, time and precursor on the precipitation of Cu₃N and the formation of secondary phases of Cu and Cu₂O are surveyed, with emphasis on shape and size control. Furthermore, Cu₃N nanostructures possess excellent optical properties, including a narrow bandgap in the range of 0.2 eV–2 eV for visible light absorption. In that regard, understanding the effect of the electronic structure on the bandgap and on the optical properties of Cu₃N is therefore of interest. In fact, the density of states in the d-band of Cu has an influence on the band gap of Cu₃N. Moreover, the potential of Cu₃N nanomaterials for photocatalytic dye-degradation originates from the presence of active sites, i.e., Cu and N vacancies on the surface of the nanoparticles. Plasmonic nanoparticles tend to enhance the efficiency of photocatalytic dye degradation of Cu₃N. Nevertheless, combining them with other potent photocatalysts, such as TiO₂ and MoS_2, augments the efficiency to 99%. Finally, the review concludes with perspectives and future research opportunities for Cu₃N-based nanostructures.

The Chemistry of Cu3 N and Cu3 PdN Nanocrystals

The precursor conversion chemistry and surface chemistry of Cu₃ N and Cu₃ PdN nanocrystals are unknown or contested. Here, we first obtain phase-pure, colloidally stable nanocubes. Second, we elucidate the pathway by which copper(II) nitrate and oleylamine form Cu₃ N. We find that oleylamine is both a reductant and a nitrogen source. Oleylamine is oxidized by nitrate to a primary aldimine, which reacts further with excess oleylamine to a secondary aldimine, eliminating ammonia. Ammonia reacts with Cu^I to form Cu₃ N. Third, we investigated the surface chemistry and find a mixed ligand shell of aliphatic amines and carboxylates (formed in situ). While the carboxylates appear tightly bound, the amines are easily desorbed from the surface. Finally, we show that doping with palladium decreases the band gap and the material becomes semi-metallic. These results bring insight into the chemistry of metal nitrides and might help the development of other metal nitride nanocrystals.

Precursor chemistry of metal nitride nanocrystals

Metal nitride nanocrystals are a versatile class of nanomaterials. Depending on their chemical composition, the optical properties vary from those of traditional semiconductor nanocrystals (called quantum dots) to more metallic character (featuring a plasmon resonance). However, the synthesis of colloidal metal nitride nanocrystals is challenging since the underlying precursor chemistry is much less developed compared to the chemistry of metal, metal chalcogenide or metal phosphide nanocrystals. Here, we review chemical approaches that lead (or could lead) to the formation of colloidally stable metal nitride nanocrystals. By systematically comparing different synthetic approaches, we uncover trends and gain insight into the chemistry of these challenging materials. We also discuss and critically evaluate the plausibility of certain suggested mechanisms. This review is meant as a guide for the further development of colloidal nitride nanocrystals.

Synthesis of Cu3N and Cu3N-Cu2O multicomponent mesocrystals: non-classical crystallization and nanoscale Kirkendall effect

Mesocrystals are superstructures of crystallographically aligned nanoparticles and are a rapidly emerging class of crystalline materials displaying sophisticated morphologies and properties, beyond those originating from size and shape of nanoparticles alone. This study reports the first synthesis of Cu₃N mesocrystals employing structure-directing agents with a subtle tuning of the reaction parameters. Detailed structural characterizations carried out with a combination of transmission electron microscopy techniques (HRTEM, HAADF-STEM-EXDS) reveal that Cu₃N mesocrystals form by non-classical crystallization, and variations in their sizes and morphologies are traced back to distinct attachment scenarios of corresponding mesocrystal subunits. In the presence of oleylamine, the mesocrystal subunits in the early reaction stages prealign in a crystallographic fashion and afterwards grow into the final mesocrystals, while in the presence of hexadecylamine the subunits come into contact through misaligned attachment, and subsequently, to some degree, realign in crystallographic register. Upon prolonged heating both types of mesocrystals undergo chemical conversion processes resulting in structural and morphological changes. A two-step mechanism of chemical conversion is proposed, involving Cu₃N decomposition and anion exchange driven by the nanoscale Kirkendall effect, resulting first in multicomponent/heterostructured Cu₃N-Cu₂O mesocrystals, which subsequently convert into Cu₂O nanocages. It is anticipated that combining nanostructured Cu₃N and Cu₂O in a mesocrystalline and hollow morphology will provide a platform to expand their application potential.

Solution/Ammonolysis Syntheses of Unsupported and Silica-Supported Copper(I) Nitride Nanostructures from Oxidic Precursors

Herein we describe an alternative strategy to achieve the preparation of nanoscale Cu3N. Copper(II) oxide/hydroxide nanopowder precursors were successfully fabricated by solution methods. Ammonolysis of the oxidic precursors can be achieved essentially pseudomorphically to produce either unsupported or supported nanoparticles of the nitride. Hence, Cu3N particles with diverse morphologies were synthesized from oxygen-containing precursors in two-step processes combining solvothermal and solid-gas ammonolysis stages. The single-phase hydroxochloride precursor, Cu2(OH)3Cl was prepared by solution-state synthesis from CuCl2·2H2O and urea, crystallising with the atacamite structure. Alternative precursors, CuO and Cu(OH)2, were obtained after subsequent treatment of Cu2(OH)3Cl with NaOH solution. Cu3N, in the form of micro- and nanorods, was the sole product formed from ammonolysis using either CuO or Cu(OH)2. Conversely, the ammonolysis of dicopper trihydroxide chloride resulted in two-phase mixtures of Cu3N and the monoamine, Cu(NH3)Cl under similar experimental conditions. Importantly, this pathway is applicable to afford composite materials by incorporating substrates or matrices that are resistant to ammoniation at relatively low temperatures (ca. 300 °C). We present preliminary evidence that Cu3N/SiO2 nanocomposites (up to ca. 5 wt.% Cu3N supported on SiO2) could be prepared from CuCl2·2H2O and urea starting materials following similar reaction steps. Evidence suggests that in this case Cu3N nanoparticles are confined within the porous SiO2 matrix.

Advanced Inorganic Nitride Nanomaterials for Renewable Energy: A Mini Review of Synthesis Methods

Inorganic nitride nanomaterials have attracted widespread attention for applications in renewable energy due to novel electrochemical activities and high chemical stabilities. For different renewable energy applications, there are many possibilities and uncertainties about the optimal nitride phases and nanostructures, which further promotes the exploration of controllable preparation of nitride nanomaterials. Moreover, unlike conventional nitrides with bulk or ceramic structures, the synthesis of nitride nanomaterials needs more accurate control to guarantee the target nanostructure along with the phase purity, which make the whole synthesis still a challenge to achieve. In this mini review, we mainly summarize the synthesis methods for inorganic nitride nanomaterials, including chemistry vapor deposition, self-propagation high-temperature synthesis, solid state metathesis reactions, solvothermal synthesis, etc. From the perspective of nanostructure, several novel nitrides, with nanostructures like nanoporous, two-dimensional, defects, ternary structures, and quantum dots, are showing unique properties and getting extensive attentions, recently. Prospects of future research in design and synthesis of functional inorganic nitrides are also discussed.

Rational design of Cu3PdN nanocrystals for selective electroreduction of carbon dioxide to formic acid

The selective electrochemical reduction of CO₂ yields value-added products that are important renewable energy resources for carbon recycling. In this study, Cu₃PdN nanocrystals (NCs) exhibited higher electrocatalytic activity for carbon dioxide (CO₂) reduction to formic acid (HCOOH) than as-prepared Cu₃N and Cu₃Pd NCs. In addition, the reaction yielded small amounts of CO (<5%), H₂, and HCOOH as the main products, and the electrocatalytic activity of the Cu NCs was significantly enhanced by modification with N and Pd. This work demonstrates a simple and effective strategy for improving the electrochemical reduction of CO₂.

Elucidating the effect of precursor decomposition time on the structural and optical properties of copper(i) nitride nanocubes

To study the effect of time on the colloidal synthesis of Cu₃N nanoparticles, copper(ii) nitrate was thermally decomposed at 260 °C for up to 60 min in octadecylamine as a stabilizing ligand. Thermolysis of the nitrate followed four steps which included; nucleation, growth, ripening and decomposition. At 5 min, partially developed nanocubes were found in a dense population of Cu₃N nuclei. Well-defined Cu₃N nanocubes were obtained at 15 min with no presence of the nuclei. TEM images showed disintegration of the cubes at 20 min and as time progressed, all the Cu₃N decomposed to Cu by 60 min. The formation of the Cu₃N nanocubes was confirmed by XRD and XPS. FTIR suggested the formation of a nitrile (RCN) as a result of the thermal decomposition in octadecylamine (ODA) and this was confirmed using NMR and hence, a reaction mechanism was then proposed. The optical properties of the as-synthesized Cu₃N were studied using UV-vis and photoluminescence spectroscopies. The absorption spectra for particles synthesized from 5 min to 15 min showed a singular exciton peak while from 20 min to 60 min two peaks were observed. The two peaks may both be associated with the two direct transitions observed in Cu₃N or the more red-shifted peak could be a result of localized surface plasmon resonance due to the Cu nanoparticles. Nevertheless, similar to other studies, it is clear that the optical properties of Cu₃N are complex.

To study the effect of time on the colloidal synthesis of Cu₃N nanoparticles, copper(ii) nitrate was thermally decomposed at 260 °C for up to 60 min in octadecylamine as a stabilizing ligand.

Cu3N Nanocubes for Selective Electrochemical Reduction of CO2 to Ethylene

Understanding the Cu-catalyzed electrochemical CO₂ reduction reaction (CO₂RR) under ambient conditions is both fundamentally interesting and technologically important for selective CO₂RR to hydrocarbons. Current Cu catalysts studied for the CO₂RR can show high activity but tend to yield a mixture of different hydrocarbons, posing a serious challenge on using any of these catalysts for selective CO₂RR. Here, we report a new perovskite-type copper(I) nitride (Cu₃N) nanocube (NC) catalyst for selective CO₂RR. The 25 nm Cu₃N NCs show high CO₂RR selectivity and stability to ethylene (C₂H₄) at -1.6 V (vs reversible hydrogen electrode (RHE)) with the Faradaic efficiency of 60%, mass activity of 34 A/g, and C₂H₄/CH₄ molar ratio of >2000. More detailed electrochemical characterization, X-ray photon spectroscopy, and density functional theory calculations suggest that the high CO₂RR selectivity is likely a result of (100) Cu(I) stabilization by the Cu₃N structure, which favors CO-CHO coupling on the (100) Cu₃N surface, leading to selective formation of C₂H₄. Our study presents a good example of utilizing metal nitrides as highly efficient nanocatalysts for selective CO₂RR to hydrocarbons that will be important for sustainable chemistry/energy applications.

Pyridine-based low-temperature synthesis of CoN, Ni3N and Cu3N nanoparticles

CoN, Ni₃N and Cu₃N nanoparticles were prepared via low-temperature, oxygen-free liquid-phase synthesis in refluxing pyridine. This approach, leading to high-purity, narrow-size (3–5 nm) nitrides, can be generally very promising for obtaining nanosized nitrides and to address their material properties.

Synthesis and characterization of Cu3N nanoparticles using pyrrole-2-carbaldpropyliminato Cu(ii) complex and Cu(NO3)2 as single-source precursors: the search for an ideal precursor

Herein, we report the synthesis and characterization of Cu₃N nanocrystals using two single-source precursors, bis (pyrrole-2-carbalpropyliminato) Cu(ii) (PPC) and Cu(NO₃)₂·3H₂O.

Influences of nitrogen partial pressure on the optical properties of copper nitride films

Cu₃N films were prepared by radio frequency magnetron sputtering techniques and the optical properties of the films were investigated.

Synthesis of Ni3N/SiO2 nanocomposites and investigation of their intrinsic static and dynamic magnetic properties

Ni₃N/SiO₂ nanocomposites were prepared via nitriding of a Ni/SiO₂ nanocomposite precursor in flowing ammonia.