Nanomaterials are materials measured in nanometers (10-100nm), which is a millionth of a millimetre. For reference, the width of a human hair is around 50,000 nanometers. Today, nanomaterials are employed to solve some of our biggest challenges, like cleaning up pollution or monitoring our health. Scientists are also increasingly becoming skilled at designing them for specific jobs.
One area where Nanocomposite materials, which are nanomaterials made of 2 or more materials, are used is to create photocatalysts. These are nanocomposites that can harness the power of light. Instead of generating electricity, however, they use light energy to kickstart chemical reactions. Nanocomposites can also play roles in generating energy or even acting as super-sensitive detectors. Researchers are constantly exploring new recipes for these nanocomposites, aiming to make them more efficient, affordable, and versatile.
In a recent study published in Scientific Reports, a team of researchers from the Manipal Institute of Technology and S.E.A College of Engineering and Technology has successfully created a promising new nanocomposite material with a dual use: it's both an effective photocatalyst for cleaning up dyes and a sensitive chemical sensor.
They focused on two common metal oxides: copper oxide (CuO) and zinc oxide (ZnO). Copper oxide is known for its potential in various applications, including catalysis. Zinc oxide, perhaps familiar from sunscreen lotions, also has interesting electronic properties. The researchers added small, controlled amounts of ZnO into the CuO structure, a process called doping. They created several versions, mixing CuO with 1%, 3%, or 5% ZnO, creating CuO-ZnO nanocomposites.
To synthesise these, the researchers used a commonly used method called solution combustion synthesis, which is like a very rapid, controlled chemical bonfire at the nanoscale. Using techniques like X-ray diffraction (XRD), they confirmed they had successfully created tiny, well-formed crystals of the CuO-ZnO nanocomposite, measuring only about 23 nanometers across on average. Scanning electron microscopy (SEM) showed that the particles formed porous, somewhat irregular shapes. This irregularity was found to be beneficial as it provides more surface area for reactions to happen.
The researchers then tested the capabilities of the different mixtures. First, they explored its photocatalytic properties by breaking down unwanted pollutants in water. They chose two common industrial dyes, Congo Red (CR) and Methylene Blue (MB), which are notorious water pollutants, often difficult to remove. They mixed a small amount of their best-performing nanocomposite (the one with 5% ZnO doping) into water contaminated with these dyes and exposed the mixture to UV light, simulating sunlight. Over 120 minutes, the nanocomposite managed to break down, or decolorise, 71.35% of the Congo Red and 53.35% of the Methylene Blue.
The nanocomposite uses the UV light's energy to generate highly reactive molecules, like hydroxyl radicals (OH•) and superoxide radicals (O₂⁻•), which attack and dismantle the complex dye molecules. The study found that the amount of catalyst mattered; adding more catalyst, up to a certain point, sped up the cleaning process, but not beyond. This works because the material absorbs photons (light particles) with enough energy to excite its electrons, leaving behind holes. This energy difference needed is called the band gap, which they measured to be between 2.1 and 2.5 electron Volts (eV) for their materials – a range suitable for absorbing UV light. The excited electrons and holes then drive the formation of those reactive agents.
The team also investigated the material's potential as a chemical sensor to see if it could detect specific molecules. They focused on glucose (blood sugar) and ascorbic acid (Vitamin C). They incorporated the nanocomposite into a carbon paste electrode, a standard setup for electrochemical sensing, and dipped it into solutions containing varying amounts of glucose or ascorbic acid. By applying a voltage and measuring the resulting electrical current, they observed distinct changes in the electrical signals that corresponded directly to the concentration of glucose or ascorbic acid present. Once again, the CuO+5%ZnO version proved remarkably effective, showing apparent oxidation and reduction peaks (electrical signatures) that shifted predictably as the concentration of the target chemical changed.
While both CuO and ZnO have been studied individually, combining them to create ZnO-doped CuO nanocomposites is novel. Doping CuO with ZnO strategically combines a p-type semiconductor, CuO, which conducts electricity via positive holes, with an n-type semiconductor, ZnO, which conducts via negative electrons. This junction between the two types can potentially improve the separation of the light-generated electrons and holes, making the photocatalysis more efficient.
The results suggest improved performance compared to what might be expected from pure CuO alone, showcasing the benefit of this doping strategy. However, the study also points out that when testing the photocatalyst's reusability over five cycles, they noticed a decline in its effectiveness, although it still retained some activity. They speculated that intermediate products formed during the dye breakdown might gradually clog up the active sites on the nanoparticle surface, like microscopic grime building up on the cleaning machinery. Further research could focus on improving this long-term stability.
The development of multi-functional nanomaterials holds significant promise. Materials that can efficiently break down industrial dyes using just light could lead to cheaper and more effective wastewater treatment methods, helping to combat water pollution from textile and other industries. Simultaneously, the ability of these same materials to act as sensitive chemical sensors opens doors for developing better diagnostic tools. These tiny dots could pave the way for developing more efficient, affordable, and sustainable technologies to address pressing global needs in both environmental protection and healthcare.
This research article was written with the help of generative AI and edited by an editor at Research Matters.
Editor's Note: A typo was rectified. The error is regretted.