Copper Oxide Nanostructures Achieve Oil-Water Separation Without Organic Modifiers

A single-step synthesis that turns ordinary copper oxide into a water-repelling, oil-loving material could reshape how industries clean up oil spills and separate stubborn oil-water mixtures. Writing in Scientific Reports, Elmira Velayi of Azarbaijan Shahid Madani University reports the fabrication of hierarchical, urchin-like copper oxide nanostructures that achieve extreme wettability without any of the low-surface-energy organic modifiers that most superhydrophobic materials require. The resulting powders and coatings combine a static water contact angle of 161.1 degrees with complete oil wetting, a combination that allows them to soak up oil from emulsions while shedding water almost entirely. The work, published open access under a Creative Commons license, arrives at a moment when the environmental and economic costs of oil contamination, from marine spills to industrial wastewater, continue to drive demand for cheaper, more durable separation technologies.

The central innovation lies in what the synthesis leaves out. Conventional routes to superhydrophobic surfaces typically follow two steps: first, engineers build microscopic roughness into a material, and second, they chemically treat it with fluorinated silanes, fatty acids, or other organic compounds that lower the surface energy and prevent water from sticking. Those modifiers add cost, complexity, and in some cases environmental concerns of their own, since fluorinated chemicals in particular raise questions about persistence and toxicity. Velayi’s approach instead relies on the intrinsic chemistry of copper oxide itself. By growing hierarchical urchin-like CuO structures through a facile one-step method, the study shows that the surface can reach superhydrophobicity through a subtle mechanism: hydroxylation of the copper oxide surface reduces the polar component of its surface free energy. Combined with the pronounced micro- and nanoscale roughness of the urchin morphology, that reduction is enough to push the surface past the superhydrophobic threshold.

The physics behind this behavior is the interplay between roughness and surface chemistry described by classical wetting theory. When a water droplet rests on a textured surface, it can either wet the grooves completely, as on a glass window, or sit atop the texture on a cushion of trapped air, a state known as the Cassie-Baxter regime. In the latter case, the apparent contact angle increases dramatically, and droplets roll off at slight tilts, picking up dirt particles as they go. The reported static water contact angle of 161.1 degrees, with a measurement uncertainty of about 2 degrees, places the CuO-coated mesh firmly in this regime, where only a tiny fraction of the droplet’s base actually touches the solid. Oil, by contrast, has far lower surface tension than water, so it spreads readily across the same textured surface, giving an oil contact angle of 0 degrees. A material that repels water yet welcomes oil is precisely what a selective oil-water separation filter needs.

To demonstrate the practical payoff, the researchers grew the hierarchical CuO structures in situ on a stainless-steel mesh, converting an everyday filtration material into a superhydrophobic, superoleophilic membrane. When an immiscible mixture of chloroform and water was poured onto the coated mesh, the chloroform, a dense oil, percolated through while the water was retained above the surface. The separation efficiency reached approximately 98 percent, and the chloroform permeation flux measured about 7300 liters per square meter per hour, a throughput figure that matters enormously at industrial scale, where slow filters become bottlenecks. Gravity-driven membranes with fluxes in this range can process large volumes quickly, and the fact that the coating was grown directly on the mesh rather than sprayed or glued on suggests good mechanical integration, an important consideration for filters that must survive repeated use, cleaning cycles, and abrasive slurries.

The mesh filter addresses free oil floating on water, but many real-world contamination problems are harder: oil-in-water emulsions, in which droplets of oil are dispersed so finely that they pass straight through ordinary filters. For this challenge, the study evaluated the superhydrophobic CuO powders as selective adsorbents. Because the particles attract oil and repel water, they preferentially capture oil droplets from the emulsion, and the reported oil adsorption selectivity exceeded 19, meaning the material absorbed far more oil than water by a wide margin. The paper also points toward deployment in the form of sorbent booms and pillows, the familiar containment tools used to soak up oil slicks at sea. A sorbent made from a material that strongly rejects water can hold more of its capacity for oil rather than wasting it on seawater, which is a persistent weakness of conventional polypropylene booms that gradually take up water alongside the oil they collect.

Copper oxide brings additional advantages that make the result more than a laboratory curiosity. Copper and its oxides are abundant, comparatively inexpensive, and supported by mature industrial chemistry, so scaling production of CuO powders and coatings is far less daunting than scaling exotic fluorinated treatments or rare-metal surfaces. The one-step synthesis is itself a simplification: fewer processing stages mean lower energy input, less solvent handling, and fewer opportunities for batch-to-batch variability. The absence of organic modifiers also removes a common failure mode, since organic monolayers on superhydrophobic surfaces can be stripped by abrasion, UV exposure, or solvents, silently degrading performance. A surface whose water repellence arises from its own oxide chemistry and geometry has, in principle, a more robust foundation, although long-term durability testing under mechanical wear and chemical exposure remains an essential next step for any such coating.

The environmental context sharpens the significance of these numbers. Oil spills release millions of liters of petroleum into oceans and waterways, and even routine industrial operations, from machining and metalworking to petrochemical refining, generate oily wastewater that must be treated before discharge. Separation technologies today span gravity skimmers, dissolved-air flotation, centrifuges, and polymeric membranes, each with trade-offs among cost, throughput, selectivity, and fouling resistance. Membranes that combine high flux with high efficiency are particularly attractive because they can be compact and passive, requiring no external pressure beyond gravity. A 98 percent separation efficiency with a flux near 7300 liters per square meter per hour, achieved with a coating grown on commodity stainless-steel mesh, suggests a route to filters that are both effective and cheap enough to deploy widely, including in settings with limited technical infrastructure.

There are, as with any laboratory demonstration, gaps between the bench and the open ocean. The reported experiments used chloroform as a model heavy oil, and real crude oils, diesel, and lubricants span a wide range of viscosities, densities, and surface tensions, all of which influence how quickly a superoleophilic membrane transmits them and how completely an adsorbent soaks them up. Emulsion droplet sizes in industrial wastewater can vary by orders of magnitude, and surfactants, which stabilize many emulsions, can also adsorb onto and alter the wettability of oxide surfaces. The study’s evidence for the mechanism, the synergy between hierarchical roughness and hydroxylation-driven reduction of the polar surface energy component, is consistent with the measured contact angles, but translating that understanding into predictions for other oils and other oxide systems will require further work. Recovery of absorbed oil from the powders, and regeneration of the adsorbent for reuse, are likewise practical questions that determine lifecycle economics.

Even so, the conceptual contribution is clear and potentially influential: superhydrophobicity does not always require fluorinated chemistry or elaborate organic grafting. By tuning the surface energy of an oxide through hydroxylation and pairing it with the right hierarchical texture, a common, inexpensive material can be pushed to the extremes of the wetting spectrum. That principle could extend beyond copper oxide to other transition-metal oxides, and beyond oil-water separation to anti-icing coatings, self-cleaning surfaces, and corrosion protection, all fields where the environmental profile of fluorinated modifiers has come under scrutiny. The work also underscores how much performance can be extracted from geometry alone; the urchin-like hierarchy of the CuO structures does much of the heavy lifting, amplifying a modest chemical effect into a dramatic macroscopic one.

For now, the study offers a compelling proof of concept across three complementary applications: a high-flux mesh filter for immiscible oil-water mixtures, a selective powder adsorbent for oil-in-water emulsions, and a candidate material for spill-response booms and pillows. The combination of a one-step, modifier-free synthesis, near-complete oil wetting, water contact angles above 160 degrees, 98 percent separation efficiency, and oil adsorption selectivity greater than 19 makes a strong case that copper oxide nanostructures deserve a place in the toolkit of water treatment engineers. As the peer-reviewed, open-access article by Velayi in Scientific Reports makes available to any laboratory with the relevant equipment, the barrier to reproducing and building upon the work is low, which is often the precondition for a materials discovery to travel from the journal page to the spill site.

Subject of Research: Modifier-free superhydrophobic copper oxide nanostructures for oil-water separation

Article Title: Fabrication of superhydrophobic and superoleophilic copper oxide nanostructures without organic modifiers for oil–water separation

Article References: Velayi, E. (2026). Fabrication of superhydrophobic and superoleophilic copper oxide nanostructures without organic modifiers for oil–water separation. Scientific Reports. https://doi.org/10.1038/s41598-026-75307-4

Image Credits: AI Generated

DOI: 10.1038/s41598-026-75307-4

Keywords: superhydrophobic, superoleophilic, copper oxide, oil-water separation, nanostructures, wettability, oil spill cleanup, membrane filtration, oil-in-water emulsion, surface free energy, stainless-steel mesh, adsorbent