Abstract Gas sensors have been fabricated based on fieldionization from titanium oxide nanotubes grown on tita-nium foil. Ordered nanaotube arrays of titanium oxides weregrown by the anodization method. We measured breakdownvoltages and discharge currents of the device for variousgases. Our gas ionization sensors (GIS) presented good sen-sitivity, selectivity, and short response time. The GISs basedon TiO2 nanotube arrays showed lower breakdown voltage,higher discharge current, and good selectivity. An excellentresponse observed for Ar compared to other gases. Besides,by introducing 2 % CO and 4 % H2 to N2 flow gas, theamount of breakdown voltage shifts about 20 and 70 volts tothe lower values, respectively. The GIS works at room tem-perature and has the ability of detect inert gases with highstability and good linearity. Besides, short response time ofabout 1 second for the GISs based on TiO2 nanotube arraysmakes them excellent for gas sensing applications. Sharpedges of the nanotubes, through enhancing the applied elec-tric field, reduce operating voltage to the reasonable valuesand power consumption.
A. NikfarjamFaculty of New Sciences & Technologies, University of Tehran,P.O. Box 14395-1561, Tehran, Iran
R. Mohammadpour · A. Iraji zad (B)Institute for Nanoscience and Nanotechnology,Sharif University of Technology, Azadi Street,P.O. Box 14588-89694, Tehran, Irane-mail: [email protected]: +98-21-66005410
A. Iraji zadDepartment of Physics, Sharif University of Technology,Azadi Street, Room 436, P.O. Box 11365-9161, Tehran, Iran
1 Introduction
Since the discovery of gas sensitivity of metal oxides, manyresearchers have developed various metal oxides as a gassensor due to the variation of their electrical conductance byexposure to gases [1–5]. Although this method has presentedrather high sensitivity, it introduces several limitations suchas the inability to detect gases with low adsorption energy.It is also challenging to use electrical conductance measure-ments to distinguish between gases in a mixture; gases withdifferent concentrations could produce output signals thesame as that for a single pure gas. Also, chemoresistors aresensitive to environmental conditions like temperature andgas flow velocity. Besides, chemisorptions could cause irre-versible changes in conductivity. We expect that using TiO2
nanotubes in GIS overcomes these disadvantages. TiO2 na-notubes based GISs are supposed to show good sensitivityand selectivity for all gases including inert types, which aredifficult to detect by chemoresistor sensors. They also showlow effectivity by environmental factors such as tempera-ture and gas flow. Therefore, they exhibit several practicaladvantages over previously reported resistive systems.
GISs are physical systems that monitor the electrical cur-rent passing through the gas between two electrodes (a probeand a sensor), which are biased at sufficient voltage. Thiscurrent is a fingerprint to distinct surrounding gases [6–12].At a constant temperature and pressure, every gas displays aunique breakdown characteristic.
The theory of gas ionization was first introduced in 1889by Paschen [13]. His law expressed the breakdown voltageVbd, as a function of the product of gas pressure, p, andthe distance between two electrodes, d , or Vbd = f (p × d).Later, Townsend expressed this breakdown by the avalancheeffect. The ionization of gas molecules carried out by elec-trons are accelerated in a high electric field [14]. This
avalanche occurs when enough gas molecules exist betweentwo electrodes and the inter-electrode spacing is larger thanthe mean free path between collisions λ : (λ � d) [15]. InPaschen’s law, secondary electrons emitted from the bom-barded negative electrode, γ , are also considered. Break-down voltage from Paschen’s theory is
Vbr = B.P.d
ln(A.P .d) − ln[ln(1 + 1γ)]
where A and B are gas parameters and γ depends on theelectrode’s material. Besides, surface roughness and workfunction of two electrodes have strong influences on break-down behavior.
In GIS, electrons first tunnel through the surface barrierwhen the thickness of the barrier is about electron wave-length. In this step, nanocurvature of TiO2 nanotubes tipsassists the electrons to tunnel through the barrier by gen-erating a very high electric field at relatively low voltages.Then the extracted electrons accelerate to the positive elec-trode. After reaching the required energy level, (if d � λ)impact ionization occurs and produces more electrons andions. The breakdown in an air gap is generally caused by arapid increase of electrons and ions through collision withneutral gases. Therefore, the breakdown voltage is affectedby the number of charges produced by ionization, where theionization coefficient depends strongly on gas pressure andidentity [16]. Since in this technique very weak chemical re-actions occur, ionization sensors show excellent reversibil-ity, fast response, and recovery times [17]. In addition, GISscan detect gases regardless of their chemical activity andelectro-negativity, so they are excellent candidates to detectnoble gases. The disadvantages of the common GIS are theirlarge dimensions and high power consumption. In GIS, TiO2
nanotubes are used as nanosized tips due to their uniquephysical and chemical properties. Their sharp edge causesto reduce operating voltage and power consumption.
As mentioned earlier, high electric field at low voltagescan be achievable at sharp tips. The local field, F , which de-termines the tunneling barrier, is equal to F = βE = βV/d
where E is the macroscopic field, V is the applied voltagebetween the two electrodes, and d is the inter-electrode sep-aration [18]. The field enhancement factor, β , has an impor-tant role in field emission, which affects the gas ionizationthreshold. In the present paper, we introduced field ioniza-tion gas sensors using TiO2 nanotubes templates. We usedthe breakdown voltage and discharge currents to character-ize the detected gases. These sensors showed good sensitiv-ity, selectivity, and reliability, and could be used in a varietyof applications like environmental monitoring.
2 Experiment
2.1 Fabrication of TiO2 nanotube array
Self-organized and vertically oriented titanium oxide na-notube array (TNTA) were prepared by anodization of Tifoil (thickness 0.25 mm; purity 99.5 %; Alfa Aesar) in atwo-electrode electrochemical cell with a platinum foil asa cathode. The electrolyte was ethylene glycol containing0.2 vol% H2O and 0.3 M NH4F. The Ti foil was biased at60 V for 30 min at room temperature to grow a nanotubu-lar TiO2 layer. TNTAs were annealed at 520 °C for 6 h inpure oxygen with heating and cooling rates of 1 °C min−1.A LEO 1550 field—emission scanning electron microscopewas used for morphological characterization of the samples.The nanotube length and also the thickness of nanoparticularelectrodes were obtained from SEM cross-section images ofelectrodes. Figure 1 shows the top view (a) and cross view(b) of TNTAs as it has been reported in our previous papers[19, 20]. Under the anodization conditions used in this study,the internal TiO2 tube diameter is 50 nm and the wall thick-ness 20 nm, while the length could be up to 10 micrometers.
Since TiO2 nanotubes on the Ti substrate plays as an elec-trode for the field ionization gas sensor, such dense and me-chanically stable electron emitter leads to a high dischargecurrent and long life time electrode.
Figure 2a shows the X-ray diffraction (XRD) patterns ofTNTAs annealed at different temperatures. It can be seenthat the as prepared nanotubular electrodes are amorphous.The degree of nanotubes crystallinity enhances as the tem-perature increases because more amorphous regions becomecrystalline, and at 520 °C rutile and anatase phases can co-exist. By increasing the annealing temperature to 580 °C,concentration of the rutile phase increases, however, we ob-serve a decrease in the anatase crystallite size. The averagecrystallite size was determined from the full width at half-maximum (FWHM) of (101) and (110) peaks for the anataseand rutile phases, respectively, using the Scherrer equation(Fig. 2b). The average anatase crystallite size was 42 nm at500 °C and reached the maximum of 63 nm at 500 °C. Onthe other hand, the average rutile crystallite size was 12 nmat 520 °C and reached the maximum of 40 nm at 580 °C.Since it has been seen that anatase phase of TiO2 containinga small rutile crystallite has better electrical performances[21, 22]. The electrodes annealed at 520 °C have been em-ployed as electrodes for gas ionization tests.
2.2 Gas sensing set up
Figure 3 shows the fabricated experimental set up consist-ing of two electrodes, TiO2 nanotubes as the cathode and asteel bar with 1 mm2 cross section as the anode. The gapbetween these two electrodes can be controlled from 10 µmto 1000 µm by a micromanipulator.
Fabrication of gas ionization sensor based on titanium oxide nanotube arrays
Fig. 1 Top view (a) and crossview (b) of the titanium oxidenanotube array
In order to satisfy Townsend’s theory, inter-electrodespacing should be larger than the mean free path of the gasmolecule collisions, λ. We know that the relation betweenthe mean free path and pressure is
λ.p = R.T√2.d2
atom.NAv
where NAv, datom, T , and R are Avagadro’s number, the ef-fective diameter of the gas atom or molecule, temperaturein Kelvin, and the universal gas constant, respectively. Themean free path of the gas molecule, λ, at atmospheric pres-sure and room temperature conditions, for Ar and N2, is
about 0.07 µm [15]. So, we used inter-electrode spacing ofat least 7 µm to ensure that they were much longer than gasmolecule collisions, λ (d > 100 λ).
We biased the TiO2 nanotubes as the negative electrodeto enhance the emission current, lowered the working volt-age, and improved the sensitivity of the sensor.
3 Results and discussion
Before detecting the target gases, the device was first testedin air under the conditions of an ambient temperature of
25 °C, relative humidity of 40 %, zero gas flow at atmo-spheric pressure, and variable inter-electrode distance. Thevoltage was increased slowly to minimize the charging cur-rent due to device capacitances. This process continued untila breakdown occurred. Figure 4 illustrates the dependenceof current discharge on the inter-electrode spacing. As it isobvious, the breakdown voltage was reduced when the inter-electrode space decreased. Sharp edges of the nanotubes,through enhancing the applied electric field reduced operat-ing voltage to the reasonable values. Of course, this voltageis not the exact gas breakdown voltage since the total volt-age dropped partly across the series resistance of the TNTAelectrodes. No breakdown has been detected under apply-ing 500 V for space more than 60 µm. We expect that atconstant inter-electrode separation, the breakdown voltage
of the sensor be a function of the gas type and its concentra-tion. We have employed 30 µm inter-electrode space in allgas ionization tests to have low enough applying voltage andinhibit large current discharge at lower interspace.
The gas ionization sensor based on TNTAs has been em-ployed to detect the identity of inert gases, such as He, Ar,and N2. Figure 5 shows the current-voltage characteristicsof He, Ar, N2, and air at room temperature (25 °C) with30 µm inter-electrode separation. Each gas exhibits a dis-tinct breakdown characteristic: Among them, He displaysthe lowest (190 V) followed by Ar (247 V), N2 (345 V) andair (363 V); so each breakdown voltage can be associatedwith one kind of gas.
Since gas pressure, p, is related to gas density, n, by theequation of p = nkT , so the discharge current at breakdown
Fabrication of gas ionization sensor based on titanium oxide nanotube arrays
Fig. 4 The dependence of current discharge on the inter-electrodespacing at different applied voltages. The inset shows the extractedvalues of current discharge and breakdown voltage for different inter–electrode spaces
voltage defines the number of gas molecules per unit volumeavailable for conduction [23]. Therefore, the discharge cur-rent quantifies concentration of the detected gases. For de-tecting each gas, by biasing the sensor at its prebreakdowncondition, introducing the gas into the air could tolerate themeasuring current between prebreakdown current in air anddischarge current in the air-gas mixture, which shows highsensitivity. Figure 6a shows the breakdown voltage of Air-
Fig. 5 The dependence of current discharge at different applied volt-ages exposed to different inert gases, at room temperature (25 °C) with30 µm inter-electrode separation. The inset shows the extracted valuesof current discharge and breakdown voltage for different gas species
Ar and He-Air gas mixtures as a function of gas concen-trations. Obviously, the breakdown voltage increases withincreasing air concentration that can be a result of higherbreakdown voltage of air. Both cases show good linearity.
Figure 6b illustrates the stability experiment of TNTAelectrodes. The breakdown phenomena were generated con-tinuously 20 times in an Ar and N2 ambient. The fluctuationof breakdown voltages was less than 7 % of the initial val-
A. Nikfarjam et al.
Fig. 6 (a) Breakdown voltage of Ar-air and He-air gases in mixture asa function of gas concentration. (b) Fluctuation of breakdown voltagefor device in both a N2 and Ar ambient. (c) Current response of GIS tointroducing Ar gas on the TNTA electrode
ues for both conditions. The fluctuation of breakdown volt-age of Ar is less than N2 because of lower discharge currentand local deterioration of nanotubes. This shows rather goodstability of GIS. Since we usually bias the GIS at voltagesless than the breakdown voltage, the discharge current de-creases and we can expect to get much less fluctuation in areal working condition.
Fig. 7 Breakdown voltages of several different relative concentrationsof various gasesin a gas–N2 mixture at 30 µm inter-electrode separa-tion.
By biasing TNTA based GIS at 300 V, we have providedthe prebreakdown condition. Figure 6c illustrates the resultof change in the electrical current as a result of detecting Ar.The measured current of the device demonstrates that GIShas a short response and recovery time because there is nota chemical adsorption/desorption process of molecular gason the TNTAs. Our measurements show that the responsetime of GIS is about 1 s.
The nanotube ionization sensor can also monitor gas mix-tures without direct use of a chromatography arrangement.Figure 7 shows the results for several different relative con-centrations of various gases in a gas–N2 mixture.
In our samples, reducing the relative concentration ofgas in the gas-N2 mixture, increases the breakdown volt-age. This is due to the increase of N2 molecules that have ahigher breakdown voltage. For instance, by introducing 2 %CO and 4 % H2 to N2 flow gas, the amount of breakdownvoltage shift by the amount of about 20 and 70 volts to thelower values, respectively.
4 Conclusion
We prepared a TiO2 nanotubes field ionization gas sensor.Our results indicate that the TiO2 nanotube is a good candi-date for GISs with high sensitivity and selectivity for inertgases. Besides, our gas ionization sensors presented a shortresponse time of about 1 second, high stability, and good lin-earity. Also, the TiO2 nanotubes field ionization gas sensorwith a low breakdown voltage and high discharge currentwas examined for detecting hazardous and explosive gaseslike CO and H2, respectively.
Fabrication of gas ionization sensor based on titanium oxide nanotube arrays
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