Many people are familiar with titanium dioxide as an active ingredient in sunscreen. Titanium dioxide works as a UV filtering ingredient in sunscreen — it helps protect a person’s skin by blocking the absorption of the sun’s ultraviolet light that can cause sunburn and is also linked to skin cancer. Learn more about titanium dioxide and sunscreen.
TiO2 has received wide attention ever since 1972 when Fujishima and Honda discovered photocatalytical splitting of water on a TiO2 electrode under ultraviolet (UV) light. Over the past decades, TiO2 has found applications in many promising areas ranging from photovoltaics, photocatalysis, to sensors. Besides the aforementioned inherent advantages of nanomaterials, nano TiO2 materials are also nontoxic, biocompatible, photocorrosion free, and cost-effective.
Recently, many novel TiO2 nanomaterials with new composition and structure have been used for various sensors. On the basis of different sensing targets or measurement principle, the sensors can be referred to as gas sensor, optical sensor, electric sensor, environmental sensor, biosensor, etc.
Sensing Properties of Titanium Oxide
TiO2 belongs to the family of transition metal oxides. There are mainly three kinds of phase structure found in nature, commonly known as anatase (tetragonal), brookite (orthorhombic), and rutile (tetragonal), whose bandgaps are 3.2, 3.02, and 2.96 eV, respectively. Anatase and rutile have wider application because they are more stable than brookite. Rutile TiO2 has a tetragonal structure and contains 6 atoms per unit cell in which the TiO6 octahedron is slightly distorted.
The inherent oxygen vacancy in TiO2 crystal indicates that there is more plus charge from Ti as compared to minus charge from oxygen. As confirmed from the stoichiometry theory for semiconductor, this kind of crystal is electron rich and belongs to n-type semiconductor. When gas absorbs onto the TiO2 surface, it could release electrons into TiO2, leading to the increase or decrease of resistance of TiO2 materials, the typical sensing mechanism of TiO2-based gas sensor. Furthermore, the conductivity property can be modified by doping other elements (especially metal elements) into TiO2 materials. By controlling the doping pattern, such as doping dosage and heating temperature, the n-type TiO2 materials can be transformed to p-type. Different from n-type, the resistance of p-type TiO2 will increase when contacting gases.
TiO2 nanomaterials are basically biocompatible and environmentally friendly and have been frequently proposed as a prospective interface for the immobilization of biomolecules, which is another important aspect for TiO2 materials. Moreover, titanium forms coordination bonds with the amine and carboxyl groups of enzymes and maintains the enzyme’s biocatalytic activity. In addition, due to the electronaccepting character of TiO2 as dicussed above, the electrons produced by the reaction between biomolecules and analyte can be harvested by TiO2. The injected electrons can be transferred to the outer circuit, which can be used to detect the reaction. With all aforementioned merits, TiO2 is one of the most competitive candidates for biosensor.
Because a particular sensing purpose can only be achieved with a specific TiO2 propertiy, the types of TiO2 phase, composition, and nanostructural feature are decisive factors for sensor performance.
Typical TiO2-based sensors for different applications, that is, gas sensor, COD sensor, and biosensor. The working principles could be quite different for sensors of different types and purposes. However, most of the sensing processes are similar and could be simplified into four steps:
(a) Receptors specifically binding to analyte
(b) A specific chemical or biochemical reaction taking place on interface and giving rise to a signal received by transducer
(c) Signal being converted to electronic signal and amplified by detector circuit using appropriate reference and
(d) Signal being sent to computers for data processing and resulting quantity presented through an interface to operator.
To create an operation-friendly interface for different markets, some basic requirements are necessary:
1. The sensor must be highly specific to the intended target, stable under normal conditions, and comparable in performance between assays.
2. Impacts from physical parameters like stirring, pH, and temperature should be minimized, and minimal pretreatments are required.
3. The response should be accurate, reproducible, and linear over the concentration range of interest, without diluting or concentrating. It should also be free from electrical or other transducer-induced noises.
4. It is always desirable to provide real-time analysis.
5. It should be inexpensive, compact, portable, and user friendly.
Therefore, TiO2 nanomaterial synthesis, sensor assembly, and operational methods are important and certainly attract researchers’ interest.
A gas sensor is a device that detects the presence of various gases, including combustible, flammable, and toxic gases. A gas sensor is important because many gases are harmful to organic life. In all kinds of semiconductor sensors, TiO2 sensor is favored thanks to its high sensitivity, fast response, low cost, and long-term stability.
TiO2 gas sensor can detect different gases including oxidative gas (O2, NO2) and reductive gas (H2, CO, NH3, H2S, VCCs), representing a resistance increase and decrease, respectively.
Normally, the microscopic reactions between these gases and TiO2 surface are much different, the sensing mechanism is more complicated, and the sensor performance could be affected by many factors. However, the sensing mechanism can be recognized by the following two processes: receptor process and transducer process.
The receptor process involves physisorption and chemisorption processes, which occur at TiO2 surface. First, a gas molecule is absorbed on TiO2 surface through physisorption, which was determined by van der Waals and dipole interactions; second, the gas molecule is further absorbed by chemisorption via a strong chemical bond formed between the gas and the surface atoms of TiO2. During the process, the temperature tends to influence the physisorption in that the increasing temperature leads to the decrease of physisorption. On the other hand, chemical bonding could influence the chemisorption for the activation energy may influence the rate of chemisorption process. Therefore, the receptor process is determined by the physisorption and chemisorption capability together.
The transducer process involves the transportation of electrons in the semiconductor materials and transformation of electrons to outer signals. This process is also affected by the electron transfer pattern including surface-controlled, graincontrolled, and neck-controlled modes. Surface-controlled mode is generally related to the compact layer structures. Gases only affect its geometric surface other than the bulk solution; thus the sensitivity of the compact layer is mainly determined by the thin film thickness. On the contrary, all parts of the porous layer will be in contact with gases, which results in more activated sites in the porous layer. Because of this nondense contact manner, each grain possesses a surface depleted area, and current has to pass through the intergranular contacts; therefore, the sensitivity of nanostructured TiO2 is affected not only by the layer thickness, but also by the pore size and the carrier’s diffusion length.
In both the receptor and the transducer processes presented above, the vacancy on TiO2 surface plays an important role. When the film was exposed in air, oxygen will be adsorbed on these surface vacancies and form anionic oxygen. Therefore, the n-type doping density of the TiO2 surface will be reduced or even it will change into p-type, which will cause the formation of depletion region and band bending on the surface. The band bending on TiO2 surface will bring a barrier for carrier transport between the particles, and then the decrease of film conductivity. When reducing gas such as H2, CO, NH3, H2S, and VOCs is adsorbed on the surface anionic oxygen, electrons will be injected into TiO2 surface, which will reduce the depletion region and release the band bending; hence, the film conductivity will be improved. On the contrary, when oxidative gases like NO2 are adsorbed on TiO2 surface, it will gain electrons from anionic adsorbed oxygen, which will increase the depletion region, leading to the decrease of conductivity.
The conductivity change can be easily transferred into resistance signal, which is the best-known sensor output signal. In most cases, the measurement is operated at constant temperature by direct current measurement. Considering that resistance is also related to temperature and microstructure, TiO2 sensor properties depend strongly on operating temperature and the semiconductor microstructure.
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