The Metal Oxide Advantage: Protecting the Future of Offshore Energy

Written by Monica Medappa. PhD

The accumulation of marine organisms on submerged surfaces, known as biofouling, presents significant economic and structural challenges. It reduces vessel speed, raises fuel costs, and compromises the longevity and integrity of hydraulic equipment^{1, 2}. This process of biofouling (see our article, "Ocean Allies: How Sponges, Corals, and Sea Cucumbers Inspire Cleaner, Greener Anti-Fouling Coatings") begins almost immediately upon immersion, as organic molecules like proteins and lipids form a primary film that attracts microorganisms. This initial layer serves as a foundation for macro-fouling; once larger organisms take hold, the protective coating is generally considered to have failed¹.

To combat this, the industry has shifted away from harmful legacy solutions like tributyl tin (TBT), which was banned globally after it was found to cause severe reproductive issues (imposex) in marine life^3,4.

To provide a clear and focused overview of modern advancements, this article will specifically discuss the application and efficacy of metal oxide nanomaterials as a primary solution for sustainable antifouling. Modern research now focuses on integrating metal oxide nanomaterials, such as copper, zinc, titanium, and iron oxides, into coatings and membranes⁵. These nanomaterials provide a lower-toxicity alternative that leverages high hydrophilicity and antibacterial properties to deter protein adsorption and microbial attachment⁶.

Why metal oxides?

Metal oxides represent a strategic cornerstone for WaveGen Energy Private Limited (WaveGen) due to their versatile functionality in marine environments, specifically through advanced photocatalysis and controlled antifouling.

Photocatalysis is a science of employing catalyst that is utilized for speeding up chemical reactions that requires or engages light. In this process, a metal oxide is activated with either UV light, visible light or a combination of both. The photogenerated pair (e-/h+) can reduce and/or oxidize a compound adsorbed on the photocatalyst surface⁷.

Leaching: To maintain efficacy, antifouling coatings must achieve a critical release rate, defined as the leaching speed necessary to entirely prevent organism⁸. 10 µg Cu/cm²/day is widely accepted as a standard threshold for deterring most marine animals, this value is primarily indicative^{8, 9}. Actual requirements fluctuate based on species, some algae persist at higher concentrations, while barnacles may be deterred by lower ones and regional environmental⁹. For instance, in brackish environments like the Baltic Sea, organisms facing osmotic stress may be more vulnerable to biocides, potentially lowering the necessary release rate¹⁰. Despite the dominance of cuprous oxide (Cu₂O) in modern coatings, specific data on these thresholds remains limited and geographically narrow, highlighting the complexity of balancing regulatory "minimum doses" with total prevention⁸.

Nanostructure: Research indicates that the toxicity of zinc oxide is heavily dictated by its specific nanostructure and physical form⁵. When comparing different morphologies, ZnO nanoparticles (NPs) generally exhibit higher toxicity than nanorods (NRs), while nanorods integrated onto a support structure demonstrate the lowest toxic potential¹¹. This suggests that toxicity is not solely a result of Zinc ion release but is also a function of the nanomaterial's shape and stability. For example, studies¹² on the marine bivalve Mytilus edulis revealed that while NPs, NRs, and ionic zinc all cause cellular damage, the nanoparticle form exerts a significantly more potent toxic effect than the nanorod form. Furthermore, this biological impact is dose- and time-dependent, reinforcing that the physical architecture of the ZnO nanomaterial is a critical determinant of its environmental safety profile⁵.

Is Zinc Oxide the New Standard for Metal Oxide Antifouling?

At WaveGen Energy, we are actively evaluating whether Zinc Oxide (ZnO) represents the next evolution in sustainable antifouling technology.

Based on the two case studies reviewed, Zinc Oxide (ZnO) emerges as a more sustainable and effective metal oxide antifoulant compared to traditional copper-based alternatives. While these case studies highlight the potential of ZnO as a sustainable antifoulant, the field of marine nanotechnology is rapidly evolving.

1)     The first case study¹ conducted in Sevastopol Bay for a year, starting August 2020 until July 2021 evaluated a strategy using metal and metal oxide nanoparticles, such as ZnO, TiO₂, and V₂O₅, as environmentally safer alternatives to traditional biocides. These materials function through photocatalytic activity, generating reactive oxygen species (ROS) and a controlled release of ions that create a toxic microlayer specifically at the coating interface. Field tests revealed that while CuO nanoparticles provided the strongest initial protection, ZnO demonstrated superior long-term performance, successfully restricting fouling to the microbial stage (dinoflagellates and diatoms) for over a year without the transition to macro-fouling. By targeting the second stage of colonization, these nanocomposites maintain surface integrity and prevent the attachment of larger organisms like shells and macroalgae, offering a promising balance between high antifouling efficacy and minimal ecological impact.

2)     The second case study¹³ conducted in three study locations across California compared the efficacy and leaching characteristics of traditional micron-sized biocides against nanoparticle-based formulations of copper and zinc oxide. Field tests across California demonstrated that most coatings effectively reduced fouling abundance and biodiversity, successfully suppressing both native and invasive species. Notably, zinc-based coatings often performed as well as, or better than, copper-based alternatives, despite zinc's generally lower aquatic toxicity. However, the study found that nanoparticle-based coatings did not provide a distinct advantage over conventional formulations regarding leaching rates or fouling prevention. These results suggest that the chemical and physical properties of the coating matrix itself play a more dominant role in determining biocide delivery and overall performance than the particle size of the active ingredients.

We at WaveGen welcome further insights, alternative data, or suggestions on emerging metal oxide alternatives to continue refining these findings

Comparative Mechanisms: Tailoring Copper, Titanium, and Iron Oxides for Targeted Defense

For WaveGen, the strategic tailoring of these metal oxides is essential to maintaining the structural integrity of offshore assets and maximizing energy output

1)     Copper oxide (CuO):

The primary mechanism of copper oxide (CuO) nanomaterials is its potent bactericidal activity, which is directly proportional to its concentration. When copper ions are released, they interact with cellular compounds to cause growth inhibition, oxidative stress, and a decrease in essential pigments like carotenoids in organisms such as green algae⁵. However, because high concentrations can contaminate aquatic ecosystems, the best use case for copper involves embedding it into a host matrix, such as polymers or zeolites to create a "controlled release" system¹⁴. For example, hosting CuO nanosheets within Zeolite 4A creates a nanocomposite that maintains an exceptionally high antibacterial efficiency (up to 99.4%) against common marine bacteria like Pseudoalteromonas etc¹⁵.

2)     Titanium dioxide (TiO₂):

The key mechanism of Titanium Dioxide as an antifoulant is its powerful photocatalytic activity, which triggers a cytotoxic effect against marine bacteria, viruses, and algae when exposed to light. Under UV or optimized visible light irradiation, TiO₂ generates reactive species that disrupt biofilm formation and inhibit the growth of fouling organisms⁵. Because of its stable chemical properties and low cost, current research focuses on modifying its physicochemical structure to broaden its absorption range, allowing it to utilize visible light more efficiently. While TiO₂ is a promising, adaptable solution for reducing environmental pollution compared to traditional biocides, its specific toxicological impact on aquatic biota remains an area of active study to ensure its safety within marine ecosystems².

3)     Iron oxide

The key mechanism of iron oxide nanomaterials (specifically Fe₂O₃ and Fe₃O₄) as antifoulants lies in their ability to enhance surface hydrophilicity and provide antimicrobial properties through both physical and chemical pathways¹⁶. When used as nanofillers in membranes, these materials function via a sieving or adsorption mechanism to remove pollutants while significantly inhibiting the growth of bacteria and microorganisms¹⁷. A particularly effective application involves combining Fe₂O₃ with TiO₂, which creates a visible-light-activated nanocomposite. Under light treatment, these functionalized membranes resist the formation of biofilms by species like Chlorella vulgaris, preventing the flux decline typically caused by microalgal attachment pathways¹⁶.

Conclusion

The global impact of marine degradation is staggering, with steel corrosion in saltwater alone costing over $2.5 trillion annually. For a company like WaveGen, these figures underscore the vital importance of our mission: protecting the structural integrity of offshore assets while advancing sustainable energy. This review has highlighted significant advancements in zinc and polymer-based coatings, which represent the next frontier in marine protection and maintenance.

Our current utilization of steel and copper oxide (CuO) provides a robust, high-performance foundation for our wave energy converters. As scientific research confirms, the key to successful copper application lies in controlled release leveraging advanced host matrices like zeolites or polymers to maximize bactericidal efficiency while strictly managing environmental leaching. At the same time, the documented long-term success of Zinc Oxide (ZnO) and the photocatalytic potential of Titanium and Iron Oxides offer a promising roadmap for future innovation. By disrupting the early stages of biological colonization, we can significantly extend the service life of hydraulic structures and maximize energy output.

At WaveGen, we believe that the most efficient energy is that which works in harmony with the sea. Whether through our current refined copper composites or the integration of next-generation nanostructured oxides, our goal remains clear: to provide cleaner, greener energy through smarter, more durable materials.

We want to hear from you! The field of marine nanotechnology and sustainable coatings is moving faster than ever. Do you have experience with alternative metal oxides, or have you seen success with "biocide-free" physical barriers? We would love to hear your suggestions or answer any questions you have about our current coating strategies.