Browsing by Author "Garcia-Segura, Sergi"

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    A Simple and Rapid Method of Forming Double-Sided TiO2 Nanotube Arrays
    (Wiley, 2022) Conrad, Christian L.; Elias, Welman C.; Garcia-Segura, Sergi; Reynolds, Michael A.; Wong, Michael S.; Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment
    Highly ordered TiO2 nanostructures, known as nanotube arrays (NTAs), exhibit potential in various energy, chemical sensing, and biomedical applications. Owing to its simplicity and high degree of control, titanium anodization serves as the prevailing NTA synthesis method. However, the practicality of this approach is marred by sluggish and inconsistent growth rates, on the order of 10 nm min−1. Growth rates strongly depend on the electrolyte conductivity, yet most reports neglect to consider this property as a measured and controllable parameter. Here, we have systematically determined a broad set of conditions (at 60 V applied potential, elevated temperatures) that allow researchers to fabricate NTAs quickly and simply. By modulating conductivity through variation of bulk electrolyte temperature and the controlled addition of several hydroxy acid species, we achieve consistent accelerated growth up to 10 times faster than traditional methods. We find that regulating the solution conductivity within a desired region (e. g., ∼800–1000 μS cm−1) enabled the fabrication of double-sided NTA layers of around 10 μm and 90 μm NTA in 10 and 180 min, respectively.
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    Catalytic Converters for Water Treatment
    (American Chemical Society, 2019) Heck, Kimberly N.; Garcia-Segura, Sergi; Westerhoff, Paul; Wong, Michael S.; Nanotechnology Enabled Water Treatment (NEWT) Center
    Fresh water demand is driven by human consumption, agricultural irrigation, and industrial usage and continues to increase along with the global population. Improved methods to inexpensively and sustainably clean water unfit for human consumption are desired, particularly at remote or rural locations. Heterogeneous catalysts offer the opportunity to directly convert toxic molecules in water to nontoxic products. Heterogeneous catalytic reaction processes may bring to mind large-scale industrial production of chemicals, but they can also be used at the small scale, like catalytic converters used in cars to break down gaseous pollutants from fuel combustion. Catalytic processes may be a competitive alternative to conventional water treatment technologies. They have much faster kinetics and are less operationally sensitive than current bioremediation-based methods. Unlike other conventional water treatment technologies (i.e., ion exchange, reverse osmosis, activated carbon filtration), they do not transfer contaminants into separate, more concentrated waste streams. In this Account, we review our efforts on the development of heterogeneous catalysts as advanced reduction technologies to treat toxic water contaminants such as chlorinated organics and nitrates. Fundamental understanding of the underlying chemistry of catalytic materials can inform the design of superior catalytic materials. We discuss the impact of the catalytic structure (i.e., the arrangement of metal atoms on the catalyst surface) on the catalyst activity and selectivity for these aqueous reactions. To explore these aspects, we used model metal-on-metal nanoparticle catalysts along with state-of-the-art in situ spectroscopic techniques and density functional theory calculations to deduce the catalyst surface structure and how it affects the reaction pathways and hence the activity and selectivity. We also discuss recent developments in photocatalysis and electrocatalysis for the treatment of nitrates, touching on fundamentals and surface reaction mechanisms. Finally, we note that despite over 20 years of growing research into heterogeneous catalytic systems for water contaminants, only a few pilot-scale studies have been conducted, with no large-scale implementation to date. We conceive of modular, on- or off-grid catalytic units that treat drinking water at the household tap, at a community well, or for larger-scale reuse of agricultural runoff. We discuss how these may be enhanced by combination with photocatalytic or electrocatalytic processes and how these reductive catalytic modules (catalytic converters for water) can be coupled with other modules for the removal of potential water contaminants.
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    Electrochemically-active carbon nanotube coatings for biofouling mitigation: Cleaning kinetics and energy consumption for cathodic and anodic regimes
    (Elsevier, 2021) Rice, Douglas; Rajwade, Kimya; Zuo, Kuichang; Bansal, Rishabh; Li, Qilin; Garcia-Segura, Sergi; Perreault, François; Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment
    Biofouling is a major obstacle in engineered systems exposed to aqueous conditions. Many attempts have been made to engineer the surface properties of materials to render them resistant to biofouling. These modifications typically rely on passive antimicrobial or anti-adhesive surface coatings that prevent the deposition of bacteria or inactivate them once they reach the surface. However, no surface modification strategy completely prevents biofilm formation, and, over time, surfaces will be fouled and require cleaning. In this work, we demonstrate the capacity of electrochemical carbon nanotube coatings in dispersing biofilms formed on the surface. A systematic analysis of the biofilm removal kinetics in function of applied current density is made to identify the optimal current conditions needed for efficient surface cleaning. Operating the electrochemically active surface as a cathode produces superior results compared to when it is operated as an anode. Specifically, the 5.00 A m−2 and 2.50 A m−2 cathodic conditions produced rapid cleaning, with complete biofilm dispersal after 2 min of operation. Surface cleaning is attributed to the generation of microbubbles on the surface that scours the surface to remove the adhered biofilm. Energy consumption analyses indicate that the 2.50 A m−2 cathodic condition offers the best combination of cleaning kinetics and energy consumption achieving 99% biofilm removal at an energy cost of ~$ 0.0318 m−2. This approach can be competitive compared to the current chemical cleaning strategies, while offering an opportunity for a more sustainable and integrated approach for biofouling management in engineered systems.