Zinc oxide-based nanoparticles for gas sensing

Over the past six decades, nanomaterials have gained significant attention for the unique properties that stem from their tiny size, in the range of a billionth of a meter. Scientists have gotten very creative in the ways they tune nanoparticle properties by manipulating their sizes, shapes, formulations, and the many ways in which they can be combined with other materials. This led to the development of a plethora of new functional materials for applications ranging from battery materials to anti-counterfeiting ink or cancer diagnostics.

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At Nano Hybrids, we develop hybrid metal/metal-oxide based nanoparticles (NPs) for gas sensing and biotechnology. Our NPs consist of a metal core encapsulated in a metal-oxide shell. The oxide shell can be functionalized with various organic molecules, to enable their integration in devices or ensure biocompatibility. Our patented production method relies on a single solvent-free synthesis step to produce our advanced nanoparticles. Our current portfolio comprises zinc, iron and nickel-based NPs, but our production technology is compatible with almost any metal and oxide.

In this newsletter article we give a short overview on zinc oxide nanoparticles, their potential for gas sensing, and related news on our recent developments with zinc-based nanoparticles.

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*Figure 1. From left to right: gas sensors can be used in e.g. the industry; example of a chemiresistive gas sensor for the detection of toxic gases (4-series Honeywell)*

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Why is gas sensing important?

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Gas sensing is important for detecting e.g. harmful gases in a lot of applications, ranging from the chemical industry to households.^1,2 There is an increasing demand for gas sensors due to e.g. increasing air pollution, and changing regulations.²^,¹

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It is important that gas sensors are reliable and efficient, such as chemiresistive sensors.¹

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Chemiresistive sensors (figure 1) contain a material that can chemically interact with a certain target gas in a way that a measurable change in the electrical resistance is detected.¹

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The issues with such devices are poor selectivity to the target gas, slow response and recovery time, poor detection at low target gas concentrations, the need for additional heating, and a short lifetime.^1,3

Why is zinc oxide so useful and what is special about nanoparticles for gas sensing?

Zinc oxide (ZnO) is a widely used chemiresistive gas sensing material because of its suitable electronic properties, excellent chemical and thermal stability, low cost, non-toxicity, and known scalable synthesis methods.¹ Various ZnO-based gas sensors have shown to detect harmful gases such as NH₃, (NO)_x, H₂S, SO₂, CO, alcohols and ethers.¹

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The issues with traditional ZnO-based gas sensors are the required high operating temperatures and their sensitivity to the presence of water, which tends to interfere with the working mechanism of the detection of the target gas.¹

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As mentioned in the introduction, NPs provide new properties that can overcome the issues faced with traditional materials. With dimensions below 100 nm, the materials have a very high surface-to-volume ratio and additional physical quantum effects, which results in more, and possibly better, interaction with the target gas and requires lower operating temperatures.¹ The synthesis of NPs is versatile so that the gas sensing properties can be tuned accordingly. Out of all nanomaterials, NPs are the most popular due to the ease and reliability of the syntheses methods.¹

What sets our hybrid Zn-core, ZnO-shell nanoparticles apart for gas sensing?

Ammonia (NH₃) is a pungent and toxic gas, used in the chemical industry plants or refrigerators for food industry.⁴^,⁵^,⁶ It is therefore important that the levels of gas-exposure are constantly monitored to detect leaks and ensure workers’ safety.⁴^,⁵ Our recent developments have shown that our ZnO-based NPs are able to detect ammonia at very low concentrations already at room temperature, meaning the gas sensing devices would not need additional heating to operate. This would save energy, make the device safer to use, and lengthen the lifetime.¹

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Furthermore, since the core of our ZnO-based nanoparticles is metallic, synergistic effects can take place. For example, the increased conductivity can be beneficial for the design of the electrodes in gas sensors. Moreover, electronic interactions at the interface of the core and shell can alter the surface of the NPs in a way that it will be more selective towards certain gases.

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Lastly, the functionalisation of our ZnO-based nanoparticles with organic molecules facilitates the development of ready-to-use products for chemiresistive gas sensors, such as stable ink formulations.

What are our next plans with gas sensing?

In the coming six months we will be working on developing a ready-to-use hybrid nanoparticle-based product for gas sensing devices. For this we are looking for gas sensing experts that want to collaborate with us to gain validation data. If this has sparked your interest or if you have other questions or inquiries, you can contact us through the form on our website.

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