Lecturer: Dr. Manavizadeh Supervisor: Dr. Nadimi Presented by: M. H. Jalalpour September 2014 Lecturer: Dr. Manavizadeh Supervisor: Dr. Nadimi Presented

Carbon monoxide sensors with

nano-structure

Lecturer: Dr. Manavizadeh

Supervisor: Dr. Nadimi

Presented by: M. H. Jalalpour

September 2014

K.N.Toosi University of technology

Carbon Monoxide Sensors

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This presentation outline


What is Carbon Monoxide and why it’s important for us Different Kinds of CO sensors introduction and properties

Introducing some kind of semiconductor sensors

Bulk based sensors

Zinc Oxide Titania Tin Oxide

Nanotube Based sensors

Carbon Nanotube Carborundum Nanotube Silicon Nanotube

Conclusion Reviewing sensor types and comparing results

Simulation Methods


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Carbon monoxide (CO)

• Carbon monoxide consists of one carbon atom and one oxygen atom.

• Why carbon monoxide detection is so important? carbon monoxide is a colorless, odorless, and tasteless gas that is slightly less dense than air. It is toxic to humans and animals when encountered in higher concentrations.

• Carbon monoxide is not easily recognized because the signs and symptoms are similar to those of other illness

• This odorless, colorless gas can cause sudden illness and death.

• Carbon monoxide poisoning is the most common exposure poisoning in the United States.

• CO Sources : vehicle exhaust – smoking indoors – water heaters – furnaces - fireplaces


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CO sensors types

• Opto chemical• The detector consists of a pad of a

colored chemical which changes color upon reaction with carbon monoxide

• Biomimetic• works in a fashion similar to

hemoglobin• darkens in the presence of CO• seen directly or connected to an

infrared source

• Electrochemical• a type of fuel cell• current related to amount of CO

• Semiconductor


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Semiconductor gas sensors

• Semiconductor gas sensors (metal oxide sensors) are electrical conductivity sensors. The resistance of their active sensing layer changes due to contact with the gas to be detected.

• Thin wires of the semiconductor tin dioxide on an insulating ceramic base provide a sensor monitored by an integrated circuit.

• This sensing element needs to be heated to approximately 400 °C in order to operate.

• Lifespans are approximately five to 10 years.

• The large power demand of this sensor means that it is usually powered from the mains. A battery-powered, pulsed sensor is available with a lifetime in months.

• This technology has traditionally found high utility in Japan and the far east with some market penetration in USA.

• However the superior performance of electrochemical cell technology is beginning to displace this technology.


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Zinc oxide(ZnO)• Detection Process: changes in electrical current

passing through zinc oxide nanowires due to adsorption of gas molecules.

• nanorods dimensions 1–100 nm , aspect ratios 3-5.[1]

• Application of CO sensors using ZnO can be improved by coating ZnO nanorods with Carbon. Also Cr-ZnO and Cu-ZnO[1]

• Production steps: Si Substrate – Ultrasonic Cleaning – Spin Coating and Drop Casting and annealing to adding a seed layer of ZnO to the Si sub – Hydrothermal Process for Growing NRAs – again annealing by 550 °C (ZnO-550) For improving crystallinity – CVD for Carbon Coating (ZnO@C)

• CO sensing Steps: Vacuum – Nitrogen till stable pressure – CO adding to chamber – Measuring the Current in the -1V reverse. The testing temp is 400 C

SEM (scanning electron microscope ) micrographs of (A) as prepared ZnO

transmission electron microscope (TEM) photographs of cross-section of C@ZnO

[1] Chung, Ren-Jei, et al. "Preparation and Sensor Application of Carbon Coated Zinc Oxide Nanorods Array." Journal of The Australian Ceramic Society Volume 49.2 (2013): 81-88.

Zinc oxide extra links [1] Au, C. T., W. Hirsch, and W. Hirschwald. "Adsorption of carbon monoxide and carbon dioxide on annealed and defect zinc oxide (0001) surfaces studied by photoelectron spectroscopy (XPS and UPS)." Surface science 197.3 (1988): 391-401.[2] González-Vidal, J. L., et al. "CO sensitivity of undoped-ZnO, Cr-ZnO and Cu-ZnO thin films obtained by spray pyrolysis." Revista mexicana de física 52 (2006): 6-10.


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Zinc Oxide Results

ZnO@C device showed good sensitivity at 150 °C and 200 °C .

No obvious signal was observed for ZnO-550 device at 100 °C, 150 °C and 200 °C.

• CO reacting with O2, the products are free electrons and CO2 . CO would be expected to react with the adsorbed oxygen on the device surface, and then induce a change in the current

• ZnO NRAs could serve as a CO sensor at a temperature as high as 250 °C.

• The temperature is very important factor.


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• Titania - Titanium dioxide - TiO2

• The most sensitive phase : anatase. However, it can easily and irreversibly convert to the rutile phase at about 600°C to 800°C. In exhaust air pollutants monitoring, TiO2 as a gas sensor is concerned with the high operating temperature necessary the phase transformation from anatase to rutile can cause a drastic decrease of sensor sensitivity. [2]

• TiO2 with the Nb as a dopant to enhance the thermal stability. This addition can hinder the anatise to rutile phase transition.

• TEM observation showed that both Nb-doped and pure TiO2 powder had uniform morphology in the anatase structure.

• the Nb addition can inhibit the grain growth while maintaining high surface area of the powder . Beside the effect of phase transformation (thermal stability), sharp increase in sensitivity are expected when the grain size becomes smaller than the space-charge depth.

• 3% Nb–TiO2 was the optimum loading

Titania

[2] Anukunprasert, T., C. Saiwan, and E. Traversa. "The development of gas sensor for carbon monoxide monitoring using nanostructure of Nb–TiO< sub> 2</sub>." Science and Technology of Advanced Materials 6.3 (2005): 359-363.


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Titania Results• The gas sensing characteristics were examined by fixing the

concentration of CO at 1000 ppm and the operating temperature at 550 °C.

• Sensitivity was defined as the ratio of Rair/Rgas

• A step response was observed by switching the flow from air to gas and gas to air.

• Best results for pure TiO2 is at 650 °C not in 850. because in high temperatures we have anatase to rutile structure changing.


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Tin oxide• Tin Dioxide (stannic oxide). Inorganic compound

with the formula SnO2.[3]

• The combustion-generated Au-doped SnO2 materials yielded sensors with excellent sensor response and time response. [3]

• gold (Au) has been demonstrated to dramatically improve tin dioxide gas sensors in terms of sensor response and selectivity to some target gases.

• Localized gold on the surface of SnO2 can make better sensor than through SnO2 .

• Three methods were considered for generating the gold nanoparticles: combustion synthesis (CS), metal precipitation (MP) and sputtering (S)[3]

[3] Bakrania, Smitesh D., and Margaret S. Wooldridge. "The effects of the location of Au additives on combustion-generated SnO2 nanopowders for co gas sensing." Sensors 10.7 (2010): 7002-7017.


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• The sensor response was defined as S = Ra/Rg , where Ra is the resistance in air while Rg is the resistance in target gas, or CO in this case. Time response (τ) was calculated using an algorithm that evaluated the time required for the sensor to achieve 90% of the final resistance value Rg after CO exposure. [6]

• Results improved to S=11.3 and τ=51s from S=6.1 and τ=60s by changing doping structure.

Tin oxide


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Tin Oxide Results


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Ab initio Simulation methods• For discovering new technologies or investigation over new materials that the experimental

laboratories are not ready to test them , we need to use simulations. Also simulations are faster in time and cheaper in the price.

• the Hartree-Fock (HF) method is a method of approximation for the determination of the wave function and the energy of a quantum many-body system.

• Hartree Product is not suitable for antisymmetric systems and the Pauli exclude principle is not included in it.

• The Hartree-Fock method often assumes that the exact, N-body wave function of the system can be approximated by a single Slater determinant.

• DFT is among the most popular and versatile methods available in condensed-matter physics . The Hartree fock approximation usage is more on chemistry while DFT is on physics specially solid-state physics. With this theory, the properties of a many-electron system can be determined by using functionals which in this case is the spatially dependent electron density.(Thomas–Fermi model)

• The major problem with DFT is that the exact functionals for exchange and correlation are not known except for the free electron gas. However, approximations exist which permit the calculation of certain physical quantities quite accurately. In physics the most widely used approximation is the local-density approximation (LDA), that depend solely upon the value of the electronic density at each point in space

Hartree Product antisymmetric discreption Slater determinant


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CNT• Unlike the conventional metal

oxides semiconductor, which needs high operation temperature (>350 °C) for the gases detection, the CNTs can detect NO2 NH3 and O2 with fast response time and high sensitivity at room temperature.[4]

• The intrinsic SWCNTs cannot detect some gaseous molecules such as carbon monoxide (CO) due to the weak van der Waals interaction between the nanotubes’ surface and the molecules. To solve these limitations of intrinsic SWCNTs, external doping or functionalization schemes have been proposed.

• The Pd/SWCNT can be utilized as good sensors for CO molecules due to strong binding energy and large electron charge transfer between the Pd/SWCNT and this molecule

[4] Yoosefian, Mehdi, Zahra Barzgari, and Javad Yoosefian. "Ab initio study of Pd-decorated single-walled carbon nanotube with C-vacancy as CO sensor." Structural Chemistry 25.1 (2014): 9-19.


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Pd decorated SWCNT• Testing Conditions: Gaussian 03 program - HF/3–21G* basis set - effective

core potential (ECP) standard basis set LanL2DZ for Pd and the standard 3-21G* basis set for all other atoms - (5,5) armchair SWCNT containing 70 carbon and 20 hydrogen atoms .

• Four condition of testing : Parallel , Bridge , Top and Hollow site• parallel C-vacancy : In this case, the adsorption energy of Pd adsorbed on

the vacancy of the C-defective nanotube.• As the atomic radius of Pd is much larger than that of the C atom, doping

of the Pd atom to the vacancy causes deformation of the nearby hexagonal rings in the doping region


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CNT Results• properties of the Pd/SWCNT and Pd/SWCNT-V have dramatic changes after the

adsorption of CO molecule.• adsorption of CO onto Pd/SWCNT is more stable than that of CO onto Pd/SWCNT-V.


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SiCNT• Silicon carbide (SiC) or carborundum. It occurs in

nature as the extremely rare mineral moissanite.

• CO molecule can be absorbed to Si atoms on the wall of SiCNTs with binding energies as high as 0.70 eV and can attract finite charge from SiCNTs.[5]

• Simulation Properties: VASP code projector augmented wave (PAW) potentials are used to represent the interactions between valence electrons and cores - local density approximation (LDA) is adopt for the exchange-correlation functional - The electron wave functions are expanded in plane waves up to a cutoff energy of 450 eV, and a 1X1X6 mesh within the Monkhost-Pack scheme is used for Brilloiun Zone sampling - total energy convergence better than 1 meV. - All energies are calculated from relaxed structures with the maximum force, being less than 0.04 eV/ÅA supercell consisting of two basic units of a zigzag (8,0) SiCNT.

[5] Wu, R. Q., et al. "Silicon carbide nanotubes as potential gas sensors for CO and HCN detection." The Journal of Physical Chemistry C 112.41 (2008): 15985-15988.


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• Three absorption sites, that is, atop a carbon atom (C), at the center of the hexagon (h), and atop a silicon atom (Si) on the (8,0) SiCNT are considered.

• CO can be absorbed on SiCNTs at Si lattice sites, with significant binding energies and charge transfer, which could induce significant change in the electrical conductivity of SiCNTs

sites of testing

Binding energy: Eb=E(SiCNT+Mol)-E(SiCNT)-E(Mol)

SiCNT Results

Et : Charge transfer


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SiNT• Compared to carbon nanotubes (CNTs) or silicon carbon nanotubes

(SiCNTs), the SiNTs have stronger interaction with the CO and can provide more sensitive signal for CO sensing.

• (16, 0) T-C: Before gas adsorption, the (16, 0) SiNT is semiconducting with a narrow band gap . we find that CO-SiNT system show metallic property after adsorption of CO. In particular, the semiconducting (16, 0) SiNT would become metallic after adsorption CO. After the CO adsorption, the system of SiNT-CO tend to relax to the sp2 mixed hybridization structure. [6]

• Charge transfer (Qt) was obtained by counting the charge difference between the adsorbed and isolated gas molecules. The binding energy (Eb) between a gas molecule and a nanotube is defined as,

• Simulation properties: The first-principles calculations were performed by using the Vienna ab-initio simulation package (VASP).interactions were described by the projected augmented wave (PAW) method. The crystal relaxations were performed within the Generalized Gradient Approximation (GGA) with the exchange-correlation functional of Perdew and Wang

[6] Li, Kunjie, Wenchuan Wang, and Dapeng Cao. "Novel Chemical Sensor for CO and NO: Silicon Nanotube." The Journal of Physical Chemistry C 115.24 (2011): 12015-12022.


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SiNT Results• the distance between the C atom and Si atom is 1.863 Å,

similar with the Si-C bond length (1.80 Å) in the SiCNTs. The binding energy (Eb) is 1.559 eV and charge transfer (Qt) is 0.658|e|. The bond length of the CO molecule increases to 1.156 Å from 1.145 Å of free CO molecule bond. So the CO molecule can be adsorbed chemically in the T-C site with the C atom strongly bonding with a Si atom in the nanotube, while it will be weakly adsorbed on the surface of SiNT in other five configurations. That is to say, for the five configurations, CO is physically adsorbed on the SiNTs


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Conclusion

• CO detection and different CO-sensor type• Semiconductor bulk CO-sensors: Zinc Oxide , Tin Oxide

and Titinia• Nanotubes CO-sensors: CNT, SiCNT and SiNT• First Principles atomic scale modeling of nanotube

sensors• Advantage of naotube sensors verse bulk sensors• Sensitivity• Response time• Low temperature

• SiNT as the most promising nanotube for CO sensing

Thank you for your attention

Documents

Lecturer: Dr. Manavizadeh Supervisor: Dr. Nadimi Presented by: M. H. Jalalpour September 2014 Lecturer: Dr. Manavizadeh Supervisor: Dr. Nadimi Presented