Road Map of Semiconductor Metal-Oxide-Based Sensors: A Review

Abstract

Identifying disease biomarkers and detecting hazardous, explosive, flammable, and polluting gases and chemicals with extremely sensitive and selective sensor devices remains a challenging and time-consuming research challenge. Due to their exceptional characteristics, semiconducting metal oxides (SMOxs) have received a lot of attention in terms of the development of various types of sensors in recent years. The key performance indicators of SMOx-based sensors are their sensitivity, selectivity, recovery time, and steady response over time. SMOx-based sensors are discussed in this review based on their different properties. Surface properties of the functional material, such as its (nano)structure, morphology, and crystallinity, greatly influence sensor performance. A few examples of the complicated and poorly understood processes involved in SMOx sensing systems are adsorption and chemisorption, charge transfers, and oxygen migration. The future prospects of SMOx-based gas sensors, chemical sensors, and biological sensors are also discussed.

Keywords: biosensor; chemical sensor; conduction band; gas sensor; p-n junction; semiconductor metal oxides; valence band.

Figure 4

Schematic illustration of the chemical reactions at the surface of an n-type gas sensor. (a) Chemisorption of oxygen (O₂) traps electrons from the conduction band and forms the charged species atomic O⁻ and molecular O₂⁻. (b) Reducing gases (e.g., CO) react with the surface-bound oxygen and release electrons back into the crystal leading to changes in the electrical conductivity that are related to the CO concentration [159].

Figure 1

Band structure of In₂O₃ near the Brillouin zone. Here, a weak optical absorption is observed at 2.7 eV and a strong optical transition occurs between lower − lying valence bands [112].

Figure 2

Schematic illustration of (a) H₂S gas detection via hollow nanofibers and (b) the formation of the depletion layer at the p−n interface [130].

Figure 3

TiO₂ sensing mechanism shown schematically with exposure to LPG and in air. (a–c) TiO₂ sensor in air, where ionic species (O₂⁻, O⁻ and O²⁻) form due to adsorption of oxygen from ambient air on the surface film and capture the electrons from n-type TiO₂; (d–f) When LPG exposed and interacted with adsorbed oxygen, large number of electrons re-injected on TiO₂ surface and decreased the barrier hight [131].

Figure 5

Schematic view of band−bending after the ionosorption of oxygen (chemisorption), where EC, EV, and EF denote the energies of the conduction band, valence band, and Fermi level, respectively. “e⁻“ represents conducting electrons and “+” represents donor sites [13].

Figure 6

The structural and band models in the (a) presence of CO and (b) absence of CO [13,176].

Figure 7

(a) Sensing mechanism of pristine n-type SMOx materials under four conditions in the reception process: (i) formation of an accumulation layer (brown color) in the presence of reduced gas with the absence of oxygen, (ii) flat band formation in the absence of surface states due to adsorbed species, (iii) formation of a depletion layer (yellow color) in the presence of oxygen and reducing gases, and (iv) formation of a depletion layer without reducing gas in the presence of oxygen. (b) Charge transport in the sensing layer depicting tentative resistance in the transduction process [20].

Figure 8

(a) Simplified representation of the essential sensing layer components for p-type oxide semiconductor gas sensors (low), where A and C are metal–semiconductor contacts and B is a grain-grain contact of a semiconductor. The energy band diagram described by Barsan and co-workers [166]: (b,c) p-type oxide semiconductors with a simplified gas-sensing mechanism and equivalent circuit [182].

Figure 9

Sensitization mechanisms of SMOx by metal or metal oxide additives. (a) Electronic sensitization via changes in the Fermi level and (b) chemical sensitization via spillover [13].

Figure 10

Schematic view of catalytic activation based on oxidation mechanism. (a) Direct reaction on the additive surface; (b) adsorption of oxygen on SMOx additive; (c) spillover of reactive species on the SMOx surface. O = oxygen (red colour), C = carbon (blue colour) on metal nanoparticle surface [189].

Figure 11

Schematic view of spillover mechanism on a Au− loaded SnO₂ gas sensor where (a) inverse oxygen spillover and (b) oxygen spillover take place [191,197].

Figure 12

Sensor signal changes corresponding to changes in the surface charge for a SnO₂—based sensor at 300 °C [199].

Figure 13

(a) Schematic view of a SnO₂ nanowire surface in the presence of O₂ and (1) ionosorption of oxygen at defect sites of the pristine surface; (2) molecular oxygen dissociation on Pd nanoparticles followed by spillover of the atomic species onto the oxide surface; (3) capture by a Pd nanoparticle of weakly adsorbed molecular oxygen. (b) band diagram of pristine SnO₂ in the vicinity of a Pd nanoparticle. The radius of the depletion region is determined by the radius of the spillover zone [216].

Figure 14

(a) Effect of particle size on CO gas sensitivity [225]; (b) effect of particle size on H₂ gas sensitivity [217].

Figure 15

Response of Sm₂O₃ doped SnO₂ sensor for different concentrations of C₂H₂ at different relative humidity (RH) conditions [228].

Figure 16

Response of a porous ZnO gas sensor as a function of operating temperatures [230].

Figure 17

SEM images of (a) ZnO before a chemical treatment and a (b) hexagonal ZnO nanorod [236].

Figure 18

Schematic view of the classification of chemical sensors according to IUPAC [165].

Figure 19

Schematic view of the principle of chemical sensors.

Figure 20

SEM images of ZnO material at (a) low magnification and (b) high magnification. (c) Cyclic voltammetry curve of a Nafion/ZnO/Au electrode in the absence of hydrazine (solid line) and presence of 1 mM N₂H₄ (dashed line) in 0.01 M phosphate − buffered saline (PBS) (pH = 7.4). The scan rate was 100 mV s⁻¹. (d) Amperometric response of a Nafion/ZnO/Au electrode in the presence of hydrazine. The inset shows the 1/i versus 1/C plot [370].

Figure 21

(a) Schematic view of the fabrication process of the electrode; (b) (i–iv) field emission scanning electron microscopy (FESEM) images of ZnO nanorod arrays (grown without electrode) and (v–viii) ZnO nanorod arrays grown directly on the probe; (c) dynamic responses of ZnO nanorod arrays to H₂ pulses at 250 °C; (d) demonstration of the sensitivity at various temperatures [371].

Figure 22

(a) Schematic view of the fabrication of a SMOx-based chemical sensor; (b) time-dependent resistance for continuous exposure of the sensor to CO at 350 °C; (c) sensitivity of undoped and 0.76% and 1.85% co-doped ZnO sensors exposed to different concentrations of CO [372].

Figure 23

Gas-sensing performance of meso- and macroporous Co₃O₄ nanorod-based sensors. (a) ethanol sensing at different temperatures using N₂ as the reference. (b) Ethanol sensing at different temperatures using dry air as the reference. (c) gas-sensing property of porous Co₃O₄ nanorods to acetone, ethanol, and benzene at 300 °C. The sensor resistance changes in response to different concentrations of acetone, ethanol, and benzene. (d) Response of nanoparticles (NPs), meso-/macroporous nanorods (NRs), and porous plates to different concentrations of acetone, ethanol, and benzene [376].

Figure 24

Schematic view of the principle of a biosensor.

Figure 25

Mechanism of SMOx—based enzyme biosensors in three steps: (i) band energies; (ii) crystalline structure; (iii) configuration of an enzyme biosensor [32].

Figure 26

(a) Fabrication of zinc oxide nanoflowers; (b) SEM image of ZnO nanofibers; (c) cyclic voltammetry curves for a modified gold electrode without and with 100 mM glucose in PBS solution (pH = 7.0); (d) amperometric response of the ZnO nanoflower biosensor in different concentrations of glucose at 0.8 V in PBS solution (at pH = 7.0) [438].

Figure 27

In₂O₃ FET− based biosensor. (a) Images of a contacted device on an artificial eye for glucose sensing in tears. Thin-film sensor contact with the skin during tension and relaxation; (b,c) device performance of thin-film In₂O₃ on a rigid substrate and flexible artificial PDMS skin, with the transfer of In₂O₃ FETs to replicas of skin under liquid gating with PBS solution at low voltage; (d) SEM image of In₂O₃ FET device on artificial PDMS skin replica; (e) enzymatic oxidation of D−glucose to generate gluconic acid and H₂O₂; (f) representation of In₂O₃ sensors for the concentration of D-glucose in a low range of human diabetic tears and high range of blood, with the standard deviation shown in the inset [448].

Figure 28

(a) Fabrication process flow for a cholesterol biosensor (i–vi); (b) relationship between the enzyme loading, the aspect ratio of ZnO nanorods, and growth time; (c) amperometric responses at an applied potential of +0.38 V for different aspect ratios of ZnO nanorods in the presence of cholesterol; (d) calibration curves of current response versus cholesterol concentration [456].

Figure 29

Electrical characterization of amino-functionalized ZnO/PAC nanowires for AFP detection. (a) Schematic view of anti−AFP immobilization via the sandwich binding method. The anti-AFP antibody was labeled with TRITC. Fluorescence microscopy images of an (b) untreated ZnO/PAC nanowire with AFP antigen, (c) an amino-functionalized ZnO/PAC nanowire with AFP antigen, and (d) an amino-functionalized ZnO/PAC nanowire with liver carcinoma. (e) Schematic view of an electrolyte-gated ZnO/PAC nanowire-based FET. (f) Current versus potential graph of a ZnO/PAC nanowire-based FET for sequential immobilizations [478].