Cuprous oxide is also commonly known as copper oxide which is basically an inorganic compound comprising of copper and oxygen. It has some excellent properties that enable it to surpass a lot of copper compounds. They have semiconducting properties as well which enable them to possess their related applications.
There are a lot of applications of cuprous oxide as there are several different types and ways in which cuprous oxide exists. All these different types are produced after going through various processes which are all authentic in their nature. Their capabilities are all dependent upon the properties that these compounds exhibit and eventually lead up to the different applications, all of which are highly unique in their nature.
With Cu2O as its formula, Cuprous oxide or Copper (I) oxide is the inorganic compound. One of copper’s principal oxides is known as copper (I) oxide or cuprous oxide, the other being cupric oxide (CuO) or copper (II) oxide. It is a solid red color and it is a component of some of the antifouling paints. The color of this compound can be red or yellow, it is determined by the particle's size. One can find copper(I) oxide as cuprite that is a reddish mineral.
Properties of Cu2O
It is a diamagnetic solid. The oxides are tetrahedral and the copper centers are 2-coordinated in terms of their coordination spheres. In a way, there is a resemblance of the structure with SiO2’s main polymorphs, and interpenetrated lattices were featured by both of the structures.
Colorless complex [Cu(NH3)2]+ is formed by dissolving copper(I) oxide in concentrated ammonia solution and that colorless complex easily changes to the blue [Cu(NH3)4(H2O)2]2+, after being oxidized in the air. It gives CuCl-2 solutions by dissolving in hydrochloric acid. Copper (II) nitrate and copper (III) sulfate and produced by diluted nitric acid and sulfuric acid. In moist air, Cu2O degrades to the copper (II) oxide.
Cu2O is one of the materials on which there has been a lot of research in the history of semiconductor physics, and there have been demonstrations of numerous experimental semiconductor applications in this material:
-Phonoritons ("a coherent superposition of exciton, photon, and phonon").
Cu2O's lowest excitons are very long-lived. neV linewidths are used to explain the absorption lineshapes, which is the narrowest bulk exciton resonance that has ever been observed. Low group velocity is possessed by the associated quadrupole polaritons and they approach the sound’s speed. Therefore, in this medium, the movement of light is as slow as sound, leading to high polariton densities. Ground state excitons have many remarkable features and one of them is that all of the primary scattering mechanisms are quantitatively known.
Cu2O was a substance where a completely parameter-free model of absorption linewidth widening by temperature can be established, enabling the deduction of the corresponding absorption coefficient. Kramers-Kronig relations don't apply to the polaritons and Cu2O can be used to show it.
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Production of Morphological Cu2O Crystals with Various Architectures
Basic Strategies to Synthesize Faceted Cu2O Crystals
Many synthetic methods like irradiation technique, sputtering, electrodeposition, and wet-chemistry route like solvothermal synthesis, hydrothermal synthesis, and liquid reduction, can be used to prepare faceted Cu2O micro-/nanocrystals. The most broadly utilized method among those is the wet-chemistry method for manipulating Cu2O crystals’ exposure facets due to the versatile ability in the tailoring of the growth rates and nucleation along various orientations.
Gibbs-Wulff‘s law theoretically determines the crystal’s equilibrium shape. Facets of high surface energies will generally reduce from the final appearance or disappear under equilibrium conditions particularly for the high-index facets. However, under realistic conditions, the interplay between kinetics and thermodynamics results in the exposed facets and the final shapes of crystals.
According to a thermodynamic viewpoint, the inherent need to lessen the total surface energy drives the crystal’s shape-evolution during its growth process. Capping reagent’s specific facet-selective adsorption (including inorganic ion, impurity molecule, polymer, and surfactant) in a solution-phase system is an efficient method to expose various facets and reduce surface energy, leading to the appearance of a non-equilibrium Wulff construction. Capping reagent’s role in tailoring the crystal’s morphology offers a guideline for rational design and synthesizing of the Cu2O micro-/nanocrystals with required surface characteristics.
Capping Agent’s Capability and Selectivity
Basically, the distance between two adjacent undercoordinated Cu atoms on the facets and/or the density of undercoordinated Cu atoms controls the Capping agent’s capability and selectivity on various facets for a Cu2O crystal. Thus the capping agent’s choice is important in controlling the Cu2O crystals’ preserved facets. It is due to the diversities of the organic capping agents that they perform a major role in controlling Cu2O crystals shapes. For example, sodium dodecyl sulfate (SDS) and poly(vinyl pyrrolidone) (PVP) with various charges, can function as the capping agents of facets.
Formation of Crystal Facets
Also, inorganic ions can be used as capping agents to form the specific crystal facets and there have been reports of it being successful in recent years. Moreover, the exposed facets' surface energies are determined by the supersaturation of growth species during crystal growth in thermodynamics, providing a general way to produce the particular high-energy surfaces. The control of [Cu(OH)4] 2– species features can easily achieve Cu2O's shape-evolution, particularly from the simple to complex architectures.
Various facets' growth rate significantly determines the crystal's shape evolution during its nucleation and growth. Reducer's species are tuned for producing various non-equilibrium architectures which may significantly influence the growth manner and nucleation. Although, various complicated factors are always involved in the kinetic control, so there still is an unclear relationship between the kinetic factor and the facet structure. There has been a broad usage of a directional chemical etching based on crystallographic anisotropy in tailoring Cu2O architectures, and that has provided some specific benefits in the formation of Cu2O with particular surface atomic structures.
Hollow Cu2O Crystals
Being one kind of perspective architecture, there have been extensive investigations on the hollow nanostructure because of their good surface permeability for mass and charge (gas) transport, refractive index, coefficient of thermal expansion, low density, and large surface area. Thus, it is a challenge to tune the hollow structure's surface behavior accurately and a considerable scientific value is possessed by the complete understanding of the growth process and production mechanisms.
Until now, a huge amount of efforts have been made for the preparation of numerous hollow Cu2O architectures (for instance multi-shelled spheres, nano frames, and nanocages) by various growth mechanisms like Ostwald ripening, oxidative etching, and solid-state precursor transformation (including CuO and CuCl). In this article, we have briefly concluded the repeated mechanisms and approaches of attaining hollow Cu2O crystals.
Nature of the Crystals
When in presence of various interparticle boundaries, the synthesized hollow Cu2O crystals are polycrystalline basically. Although, both the structural coherence and long-range electronic connectivity are equally important in enhancing both electron mobility and high conductivity. The integration of being single and hollow crystalline shells in Cu2O crystals is thus the solution of obtaining the demand but it still is a challenge.
In comparison to the classical growth models, oriented attachment is different, which usually takes place before the Ostwald ripening process to make a hollow architecture. For instance, the production of multi-shelled Cu2O hollow spheres with a single-crystalline shell was assisted by the multilamellar vesicle and there were demonstrations of it in a CuSO4/cetyltrimethylammonium bromide (CTAB)/AA/NaOH system.
Porous Cu2O Crystals
A huge amount of attention has been gained by the porous nanomaterials with controllable pores because of their capability of interacting with the molecules, ions, and atoms but not only at the surface but in the interior too. In that sense, the best performance of these architectures is observed in mesoporous systems. Until now, there has been a successful synthesis of the porous Cu2O materials and their applicability is mainly in the fields of catalyst and dye adsorption. Thus, it is still important to fabricate and design novel porous Cu2O nanostructures with good performances and suitable pore sizes.
Initially, small nanoparticles building blocks aggregate together during Cu2O crystal’s solution-phase growth, and the aggregates evolved into the comparative stable architectures frequently through a ripening mechanism for minimizing the reaction system’s overall energy, therefore when some polymer or organic molecules are introduced, there are chances of modifications of the surface energies of building blocks. Thus, a major role would be played by tailoring the aggregation behaviors of nanoparticle building blocks in controlling the production of the porous Cu2O nanostructures.
There has been broad usage of the soft-template method for architecting porous Cu2O nanospheres. Hydroxyl group for instance functions as a capping radical and it can also cause modification in the building blocks’ aggregation manner, resulting in the production of disordered Cu2O porous nanospheres. There have been demonstrations of the β-cyclodextrin (β-CD)-driven assembly of the porous Cu2O nanospheres.
A crown-ether type architecture can be generated on the binding of the ethylene oxide in triblock copolymer’s poly(ethylene oxide) (PEO) segments in aqueous solution with the metal ions, and it is a result of dipole-ion interactions between the ethylene oxide linkages’ lone pair electron and the metal ion. Thus, copper atoms join with an oxygen atom in a hydrophilic PEO group preferentially with the help of triblock copolymers for forming short-range-ordered Cu2O mesoporous spheres.
Highly Ordered Porous Nanostructures
It should be noted that highly ordered porous nanostructures possess significant benefits because of their large surface areas as they offer more active sites for 3-dimensional connected networks and catalytic reaction for transfer of mass (like ion and molecule) from the exterior to the interior for speeding up the chemical reaction. Although, controlling the porous nanomaterials shapes via organic agent-assisted self-organization is more complicated in comparison to controlling the non-ordered porous materials shapes. Therefore, developing ordered non-spherical Cu2O porous nanostructures is still a challenge.
Cu2O Thin Films
Another significant issue to expand the application in the energy conversion is the development of the Cu2O thin films with tailored architectures. When preparing Cu2O thin films, one should necessarily consider these two main points. First is the intimate contact between substrates and the Cu2O thin films for enabling the transfer of a smooth interface charge carrier. Second is the building block’s orientation tuning in the film for maximizing the benefits. Until now, there have been various applications of numerous synthesis methods like anodic oxidation, sputtering, electrodeposition, chemical vapor deposition, and thermal oxidation for preparing Cu2O.
Electrodeposition is among the widely available methods that are a cheap and versatile method to make thin films over the conductive substrates, which can control the shapes, sizes, and orientations of the electrodeposited films efficiently by doing adjustment of the electrochemical solution conditions (for instance additive agent, solvent, species of the substrate, pH value, applied voltage, temperature, concentration, and so on).
One can easily attain Cu2O thin films with a series of symmetric dendritic morphologies and orientations, and an optimum combination of charge-transport characteristics and surface areas could be obtained, resulting in the applications in the solar energy conversion.
Distribution of Building Blocks
An enhanced electrodeposited method was used by Zhai and coworkers for specifically controlling the building blocks orientation distribution in Cu2O thin films for obtaining oriented Cu2O thin films with various faceted features and high crystallization in the citric ions’ presence at comparatively higher pH value and mild temperature.
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Applications of Cu2O
Until now, Cu2O’s main applications have been in the fields of energy conversion and environment, specifically chemical templates, sensors, and catalysts. Our main focus in this section is on the typically improved and unusual performances that are generated by tailoring hybrid Cu2O nanocomposites and Cu2O’s crystal facets. Also, there is a brief highlighting of the faceted Cu2O-templated strategy to produce well-defined hollow architectures.
According to the practical applications, including organic synthesis, CO2 reduction, water splitting, and pollutants degradation, photocatalysis usage is divided into four major areas. All these areas have employed Cu2O-based photocatalysts. There have been demonstrations of the Cu2O photocatalysts with large surface areas being efficient for pollutants’ photocatalytic degradation due to strong oxidative species’ long-time generation under solar light irradiation.
Although, when it comes to reactants, these oxidative species are less selective, resulting in photocatalyst’s poor selectivity. Other than the interface effect of hybrid Cu2O-based nanostructures, a platform is offered by the tailoring of the crystal facets to enhance the selectivity in which the reactants’ adsorption-desorption can be affected by the surface atomic structures, and the redox potential of photogenerated holes and electrons can be tuned by the corresponding electronic structures.
Photoelectrochemical Water Splitting
Solar energy can be harvested by the photoelectrochemical (PEC) solar cells for it to be converted into hydrogen fuel via water splitting. A p-type Cu2O semiconductor is of particular interest for PEC solar water splitting and hydrogen generation due to its unique features, like a 2.0 ~ 2.2 eV of direct band-gap for favorable energy band positions, good carrier mobility, and visible light absorption for PEC water splitting, with the conduction band lying +0.7 V below the hydrogen evolution potential.
There have been reports of a theoretically estimated photocurrent of -14.7 mAcm−2 with 18% corresponding efficiency for light-to-hydrogen conversion. Recently, there has been an exploration of a single Cu2O photocatalyst for hydrogen production and solar-driven water splitting. Cu2O’s stability depends on its morphology to some extent in which the stability is improved by the photogenerated carrier’s quick removal from the photocathode surface.
However, Cu2O photocatalysts can decompose water in distilled water into oxygen and hydrogen under visible light irradiation, but it is very different from the photochemical reaction in an aqueous electrolyte on the polarized Cu2O electrodes.
As the redox potentials for monovalent Cu2O’s oxidation and reduction lies within the bandgap, the usage of Cu2O for water reduction as a photocathode is electrolyte’s instability under illumination, limiting their applications in the production of solar energy. Thus, Cu2O can be a promising material in conjunction with an appropriate redox system as a p-type photoelectrode in an electrochemical photovoltaic cell. Therefore stabilization of the Cu2O photoelectrodes surface demands the usage of a conformal coating.
Carbon Dioxide’s Photo-reduction
We can satisfy the increasing needs of clean energy by carbon dioxide’s (CO2) photochemical reduction to the value-added chemicals or fuels. According to recent findings, Cu2O is a suitable option of photocatalyst for carbon dioxide’s photo-reduction that the visible light drives.
Observing the Cu2O facet's influence on the photo-reduction of CO2 was extremely interesting and according to the results, as compared to the octahedral ones, higher activity was displayed by the cuboid aggregated Cu2O. As compared to Cu2O nanobelt arrays in CO2 reduction, higher activity is possessed by the stone-like p-type Cu2O both in photoelectrochemical and electrochemical systems. Although, there is still a need for detailed investigations for uncovering the abnormal facet-dependent CO2 photo-reduction performance's underlined principles.
Cu2O-based nanostructures can significantly achieve the enhancement of carbon dioxide’s conversion efficiency. For instance, under solar light, Carbon dioxide’s photoelectrochemical reduction can be improved by the Cu2O that's anchored on the surface Cu electrode. As compared to carbon dioxide's conversion efficiency over Cu/Cu2O (n-type) electrode with the same morphology as p-type, the conversion efficiency over Cu/Cu2O (p-type) electrode is way higher.
RuOx nanoparticle's deposition on the Cu2O led to a twofold increased yield of long-lived electrons, which ends up in the improvement of carbon dioxide's visible-light-driven photo-reduction. An improved photocatalytic activity for carbon dioxide’s reduction to methanol can be displayed by the Cu2O/TiO2 heterostructure nanotube arrays that are made by an electrodeposition method. More photo-reaction active sites can be provided by the porous Cu2O/TiO2 nano junction and they can also aid in the CO2 absorption.
In past decades, there has been a huge number of investigations on gas-sensing materials to detect targeted gas, which involves the fields of human health, public safety, environmental protection, and the chemical industry. A suitable amount of deionized water was mixed with the as-tested gas-sensing material, and the above-mentioned paste was deposited with two electrodes onto a ceramic tube. Then an indirect-heated gas sensor was fabricated by putting a wired heater into the ceramic tube's center.
The gas-sensing heat system's heating current was adjusted to obtain the sensor's different operating temperatures. At last, after aging with a stable voltage at a particular relative current for a long time, gas sensing was achieved.
Until now, there has been the usage of the Cu2O crystals in the fields of chemical template, sensor template, and photocatalyst, mainly. Although, in applications like metal-insulator-metal resistive switching memories, supercapacitors, sodium-ion batteries, lithium-ion batteries, solar energy conversion, and antibacterial activity, Cu2O crystals with tailored architectures holds significance. Like that, in order to promote the above applications, tailoring Cu2O crystals facet-index for forming particular reactive surfaces is required.
For instance, there have been demonstrations of the morphology-dependent antibacterial activities by Wang and coworkers. According to the results, as compared to the cubic ones that owe to the exposed surface's different atomic arrangements, higher activity in the killing of E. coli was possessed by the octahedral Cu2O.
In addition, obvious bacteriostatic effects are shown by Cu2O which are majorly determined by their morphologies. For example, according to the findings of Guo and coworkers, as compared to the cubic ones, the octahedral Cu2O nanocrystals produce more reactive oxygen species and higher immobilization ratio, suggesting that different toxicity effects are caused to cladocerans by two morphological nanocrystals because of their dissimilarities in specific surface activities.
There were studies on the electrochemical lithiation characteristics of various polyhedral Cu2O as anodes for Lithium-ion batteries. It is expected that the usage of Cu2O with tailored architectures in these applications will result in some expected characteristics. Although applying Cu2O crystals in all of the above-mentioned fields is still under progress.
Cuprous oxide being unique in their nature due to excellent properties have remarkable applications all of which are very different from one another yet very unique in their nature. All these applications add up to the productivity of cuprous oxide as these are the things that lift the entire outgrowth of the product.
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