An Introduction to Metal Oxide Sensors

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The Future of Metal Oxide Sensors

Metal oxides (mainly II-VI semiconductors) are used as sensors due to their low cost, ease of manufacture, quick response time, wide detection ranges and resistance to harsh conditions. Metal oxide sensors are versatile semiconducting devices that detect gases, chemicals, and biological analytes by converting surface interactions into measurable electrical signals. Their key features include sensitivity, selectivity, rapid response and recovery, and long-term stability. These properties stem from the material’s structure, morphology, and electronic characteristics, which can be tuned through band gap engineering, defects, and surface modifications. As a result, metal oxide sensors offer a robust, adaptable platform for applications in environmental monitoring, industry, and healthcare.

Features of Metal Oxide Sensors

The key features of semiconducting metal oxide sensors are:

Accessing these key features are achieved via the surface properties relating to their structure, morphology and crystallinity. Metal oxides are able to carry out sensing processes including adsorption, chemisorption, charge transfer and oxygen migration. This versatility means that they can be used for sensing gases, chemicals and biological analytes.

Effective metal oxide sensors have the following specific features:

1. Charge transfer between the analyte and metal oxide
2. Measurements dependent on the analyte concentration

Why do metal oxides make good sensors?

The chemical and physical properties of metal oxides can be controlled via structural defects, morphology, grain size and specific surface area. This means that the metal oxide sensing material can be modified to meet the requirements of a given sensor. The key properties and how they can be tuned are discussed below:

Different Electronic Structures

Each metal oxide has a unique electronic structure which determines its (semi)conducting properties. Transition metal ions possess unfilled d-shells, allowing for reactive electronic transitions, wide band gaps, superior electrical characteristics and high dielectric constants. Therefore, without any intentional tuning the range of metal oxide electronic structures already gives rise to a large range in sensing capabilities.

Semiconductivity

The charge transfer properties of metal oxides arise from the large electronegativity difference and strong ionic bonding between metal and oxygen atoms. Their band gap, which can be externally excited, enables controlled charge generation and underpins sensing behavior.

In metal oxide sensors, semiconductivity governs how surface adsorption and band bending influence carrier concentration, allowing conductivity to shift measurably in response to analyte exposure. The sensing property of n-type semiconductors and p-type semiconductors depends on the energy band structure of semiconductor metal oxides.

Band Gap

When the semiconducting metal oxide thickness approaches the depletion layer width, the energy band is no longer constrained to the surface. It is now influenced by multiple grains and the electronic structure is altered which in turn impacts the movement of charge carriers.

Once the band gap energy (Eg) is reached, electrons are promoted from the valence band into the conduction band, creating mobile electrons and corresponding holes in the valence band, which both contribute to charge transport under an external electric field.

Indium oxide (In₂O₃) has small electron effective mass and exhibits a highly dispersive conduction band minimum (CBM). It has an optical band gap of 3.7 eV. Impurities introduce intra-band transitions, where electrons move between defect and ground states. By tailoring impurity size, shape, and composition, these intraband gaps can be tuned.

Semiconducting metal oxide sensors with large Eg values are thermally stable and suitable for high-temperature operation (>300 °C) of gas sensors, where a bandgap >2.5 eV is required.

Quantum confinement via miniturization also enables band gap tuning in nanostructures by applying strain within their elastic limit. This band structure governs light absorption, charge separation, and recombination, which in turn dictate semiconducting metal oxide applications in photoelectric conversion.

Stability

The electronic structure and bonding of semiconducting metal oxides often results in exceptional thermal and chemical stability. Many metal cations adopt oxidation states with filled or stable outer shells, and their strong ionic bonding with oxygen minimizes reactivity. As a result, these materials can maintain their structure and performance under elevated temperatures, oxidative environments, and repeated external stimuli.

This stability is essential for sensing, since reliable operation requires materials that can undergo continuous cycles of charge transfer, adsorption, or interaction with external fields without degrading. While less stable materials may suffer from structural breakdown or irreversible chemical changes, semiconducting metal oxides preserve their integrity, ensuring consistent, reproducible responses over time.

By combining durability with semiconducting properties that translate surface or environmental interactions into measurable electrical signals, metal oxides provide a robust platform for a wide range of sensing applications, from environmental monitoring to optoelectronic detection.

The stability of metal oxides have been enhanced via post treatments, such as annealing, calcination methods, and reduced operating temperature of sensitive materials. Metal doping and metal oxide nanomaterial hybridization have also been employed to improve stability.

Crystalline Structure

The specific crystal structure of a given metal oxide influences its physiochemical activity. For example, among the three different phases of TiO₂ films, anatase phase TiO₂ films have better sensitivity to H₂ and volatile organic compound (VOC) gases than rutile phase TiO₂ films. It is thought that longer charge carrier lifetimes in the anantase phase make it more likely for charge carriers to participate in surface reactions.

In 1991, it was shown that by reducing the crystallite size of a metal oxide induces a huge improvement in sensor performance. Nanocrystalline materials provide a significant increase in the surface/volume ratio for a material. Almost all the charge carriers in a low grain size metal oxide are trapped in surface states, with only a few thermal activated carriers available for conduction. Therefore, there is a much greater effect on sensor conductance trigger by target gases. This is due to the greater difference in charge carrier density between the activated and not activated state.

There are various types of defects in metal oxides, including point defects, line defects, plane defects, and volume defects. Physio-chemical activity can be enhanced by partial defects caused by impurities. Photoelectric activity can induce point defects, also called 0D defects. Another type of defect is a line defect, which is caused by partial crystal slides. There are two types of dislocation defects: closed rings and surface defects. There are also planar defects, which can include angular grain boundaries, stack layer faults, and twin crystals. In the crystal matrix, volume defects are voids with different structures, densities, and chemical compositions.

Property Tuning

Reducing the size of metal oxide particles to increase the surface area to volume ratio isn’t the only way to tune material properties. The introduction of noble metal, heteroatoms, and carbon materials to the metal oxide improves sensing properties due to the synergetic effect of catalyst and heterojunction. Noble metals, such as platinum and palladium, lower activation energies, enhancing catalytic activity, generating active surface sites for increased oxygen adsorption, and reducing the operating temperature of the sensor.

The significant improvement in gas sensing performance after noble metal decoration is due to two main mechanisms:

Examples of Metal Oxide Sensors

Biosensors

Metal oxides commonly detect biomaterials via catalysis or electrochemistry. Electrochemical sensors are based on a shift in potential due to the chemical interaction between biomaterials and the chemical groups such as antibodies functionalized on the metal oxide surface. Enzymatic catalysis is used to determine the concentration of biomaterials by detecting by-products, including hydrogen ions from the reaction. For example, In₂O₃ nanomaterial functionalized with glucose oxidase used enzymatic oxidation of D-glucose as the sensing mechanism. The concentration of D-glucose is estimated through the pH level determined by the concentration of hydrogen ions generated.

Photodetectors

The semiconducting properties of many metal oxides has meant they are commonly found in photodetectors. The UV sensing mechanism can be understood as the photoelectric phenomenon or the photodesorption of oxygen molecules adsorbed on the surface of metal oxides. For n-type semiconducting metal oxides such as SnO₂, ZnO, TiO₂, Zn₂SNO₄, and MoO₃ conductivity increases via the photodesorption of oxygen, induced by the illumination of the light whose photon energy is higher than the band gap. This is a measurable response used for detecting UV light.

Mechanical Sensor

Metal oxides are used to detect mechanical deformations including bending, stretching, and compression due to their piezoelectric and piezoresistive properties. ZnO is the most common material due to its excellent piezoelectric and piezoresistive performance. The microstructure of ZnO which includes crystal morphology, size, orientation, density and aspect ratio has a strong impact on its piezoresistive properties. These properties can be further enhanced through combing them with other nanomaterials such as graphene nanoplatelets or carbon hybrids.

Whilst metal oxides have been used to sense a variety of target analytes, the most common type of sensor they are found in are gas sensors:

Metal Oxide Gas Sensors

Metal oxide semiconductors have wide band gaps with tailorable electrical properties and high stability, suitable for chemiresistive gas sensors. Several semiconducting metal oxides such as tungsten oxide (WO₃), tin dioxide (SnO₂), zinc oxide (ZnO), cobalt oxide (Co₃O₄), iron oxide (Fe₂O₃), titanium dioxide (TiO₂), nickel oxide (NiO), and indium oxide (In₂O₃), have been used as gas sensing materials. Reports of metal oxide gas sensing devices include detecting a variety of toxic and inflammable gases with different detecting level capacities.

Metal oxide gas sensors typically consist of a heating layer or wire (to obtain the optimum working temperature), conducting electrodes (that measure the resistance), and sensing film which changes its resistance upon exposure.

Mechanism of Gas Sensors

The gas sensing mechanism is defined by the number of reactive surface sites and adsorption of oxygen species that further increase the number of surface reactive sites. Two processes are involved in the sensing mechanism of metal oxide gas sensors: reception and transduction.

The adsorption of oxygen and its reaction with target gases are the basis of the gas detection mechanism by using metal oxides. Oxygen adsorbs to the surface of the metal oxide by trapping electrons which increases the sensor’s resistance for n-type materials or reduces resistance for p-type materials.

Two key characteristics:

• The chemical reaction between the gas and the surface of the metal oxide.
• The equivalent changes in the electric resistance of the sensor as a result of the reaction.

Difference in sensing mechanism depends on whether the metal oxide is n-type or p-type. The receptor functions, conduction paths, and majority charge carriers results in the difference in mechanism. Adsorption of oxygen with negative charge sees:

n-type metal oxide gas sensor

N-type metal oxide semicondcutor gas sensors can detect different gases due to changes in the electrical signal caused by the gases. Gas causes changes in the electrical properties of the sensor, which are necessarily accompanied by changes in physical properties such as energy bands and work functions.

Gas sensing mechanisms for n-type metal oxides are studied at the microscale and macroscale:

The gas sensing mechanism of n-type metal oxide sensor is mostly based on the changes of resistance after they are exposed to the target gases due to the chemical interactions between the adsorbed oxygen ions and target gas molecules on the surface of materials.

When n-type metal oxide sensor is exposed to air, oxygen molecules are adsorbed by the surface of the material. The adsorbed oxygen molecules obtain electrons from the conduction band of the material, which leads to the formation of different oxygen ions, including O₂⁻, O⁻ and O²⁻.

O_2(ads) + e^- ↔ O₂^-_(ads) (< 100°C)

O₂^-_(ads) + e^- ↔ 2O^-_(ads) (100-300°C)

O^-_(ads) + e^- ↔ O^2-_(ads) (> 300°C)

Due to the decrease in electron density, an electron depletion layer is generated on the surface of the n-type MOS. As a result, a potential barrier is formed, resulting in a decrease in the conductivity of the n-type MOS. The operating temperature of the gas sensor determines the type of oxygen ion. When the sensor is exposed to the target gases, the gas molecules are adsorbed on the surface of the metal oxide sensor and react with the chemisorbed oxygen ions:

2H₂S_(gas) + O₂^-_(ads) ↔ 2H₂O_(gas) + 2SO_2(gas) + 3e^-

4NH_3(ads) + 3O₂^-_(ads) → 2N_2(gas) + 6H₂O + 3e^-

2CO_(gas) + O₂^-_(ads) → 2CO_2(gas) + e^-

2H_2(gas) + O₂^-_(ads) → 2H₂O_(gas) + 2e^-

Example of n-type metal oxides

Titanium Dioxide (TiO2) Nanoparticles

From $120

Iron Oxide (Fe₂O₃) Nanopowder

From $105

Tin (IV) Oxide Powder

From $180

Indium Oxide Powder

From $135

Tungsten Oxide Powder

From $135

p-type metal oxide gas sensor

p-type oxide semiconductors are typically less sensitive than their n-type counterparts. Instead they provide unique functionality with low humidity dependence. The ionized adsorption of oxygen on p-type oxide semiconductors triggers the formation of hole-accumulation layers (HALs). As a result, conduction occurs mainly near the surface of the HAL and not at the interparticle contact like n-type. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors.

However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration. This can be done through aliovalent doping, where atoms with a different charge (valence) than the original host atoms are used as dopants. Or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts.

The Future for Metal Oxide Sensors

Conventional metal oxide based sensors are generally fabricated on rigid substrates due to their high manufacturing temperature, which increases energy consumption and manufacturing cost as well as imposes limitations in expanding applications. However, efforts are being made to establish metal oxide sensors on flexible and stretchable substrates.

Other emerging features of metal oxide sensors include:

• n-type/p-type switching where metal oxide films can switch between preference for different charge carrier species. For example, a Pt-SnO₂ nanoporous film, an n-type material, presents tunable sensing behaviour with switching from p- to n-type toluene sensing performance as a function of the platinum content and calcination temperature.
• The enhancement of sensing performance of materials through the incorporation of secondary components. Whilst this is already an established technique for tuning material properties for a specific sensing application, the material combinations are vast and there is more to be explored.
• The advantages of interrogating sensors with alternating current rather than direct current. As using metal oxide sensors with AC instead of DC enables richer information about the sensor’s electrical and surface processes. It also reduces long-term measurement issues, and can improve sensitivity, selectivity, and stability. This makes AC-based techniques, such as impedance spectroscopy, powerful tools for both research and practical sensor applications.

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