The measurements of the Hall effects definite that the electrons have been predominant charges in the process of the conduction (which is the n-type)

COMPARISON OF ANATASE AND RUTILE TiO2 NANOSTRUCTURE FOR GAS SENSING APPLICATION

S. A. HAMDAN^*, I. M. IBRAHIM, I. M. ALI

University of Baghdad, College of Science, Physics Department, A-Jadiriyah, Iraq.

In this work, anatase and rutile titanium dioxide (TiO2) phase nanostructure have been prepared with the use of the hydrothermal approach using titanium tetrachloride solution (TiCl₃). X-ray diffraction (XRD) results of TiO₂ were proved the anatase and rutile phases observed in the films. The measurements of the Hall effects definite that the electrons have been predominant charges in the process of the conduction (which is the n-type). The morphologies of TiO₂ nanoparticles has been observed by the Field Emission Scanning Electron Microscopy (FESEM) reveal the TiO₂ rutile films nanoflower like structure, while the TiO₂ anatase films consist of voids and particles like spherical in shape. The sensitivity of the sensors was determined by varying the conductivity of the sensor material under H₂S gas with several working temperatures. It was determined that the fabricated sensor using rutile TiO₂ nanoparticle has high sensitivity, long response time, and fast recovery time compared to anatase TiO₂ nanoparticle. Fabricated sensor based rutile TiO2 nanoparticle has higher sensitivity at 373 °K.

(Received May 23, 2020; Accepted October 6, 2020)

Keywords: Titanium dioxide nanostructure, Anatase, Rutile, Gas sensors, H2S gas

1. Introduction

It is well known that Titanium dioxide TiO2 exists in 3 forms of minerals: rutile, anatase, and brookite. Anatase kind TiO2 has a structure of crystalline, corresponding to tetragon system (with the di-pyramidal habits). The Rutile Titanium dioxide kind as well has tetragon crystal structure (with the prismatic habits). Brookite kind Titanium dioxide is of an orthorhombic crystalline structure. The Titanium dioxide is one of the multipurpose materials which have applications in different products like sunscreen lotions, paint pigments, capacitors, electro- chemical electrodes, toothpastes, solar cells, [1], photovoltaic [2], photocatalysis [3], sensors, [4]

and so on. The Titanium dioxide is a wide band-gap semiconductor with a 3eV-3.20eV energy.

Functional Titanium dioxide characteristics are affected with a number of the factors, such as, particles size, phase composition, preparation technique, and surface area [5]. For the purpose of producing the particles of TiO2, a wide variety of numerous procedures of preparation like the procedure of sol-gel [6], solution combustion [7], micro-emulsion [8], chemical vapor deposition [9], and hydrothermal process [10].

The purpose of this research is utilizing hydrothermal method for synthesis of TiO2 rutile and anatase phases nanoparticles using TiCl3. Their characterization and gas sensing characteristics have been investigated. The gas sensors have been investigated as a detector for the reduction of the gas H2S.

TiO2 gas sensors are characteristic resistant-type sensors which can show a decrease or increase in resistance when probing a reductive gas or oxidative gas respectively [11, 12]. The hydrothermal technique has numerous benefits, such as the creation of a very homogeneous product of the crystalline that may be directly obtained at rather lower temperature of reaction. Its most important characteristic is that decreasing of the agglomeration between the particles, controlled particle morphology, and phase homogeneity. It as well provides uniform structure, mono dispersed particles, product purity, and control over the particles’ size and shape [13].

*

centrifuged and then washed by distilled water, this procedure has been frequent for 3 times, then with ethanol once and dried at room temperature.

TiO2 rutile phase synthesized using hydrothermal method. Initially, dissolve 0.3002M of NH2CONH2 (urea) with 20ml of distilled water and stirred for one hour at 323^oK. 4 ml of TiCl3

dissolve in 20 ml of distilled water (the acidic test is 1%) and the solution stirred and add to urea.

Mix the mixture for half an hour at 300 ^oK, then transferred into autoclaves and placed in the oven for five hours at 328 ^oK. The mixture has been centrifuged and washed by distilled water, this process has been repeated for three times, then with ethanol once.

The prepared TiO2 deposited on silicon n-type (111) and glass substrate using spin coating technique. The spin parameter was 1500 rpm for 1 minute. After deposition on the substrates was dried at 373°C in order to ensure the entire residual solvent removal.

The structure of the TiO2 anatase and rutile phase's nanostructures has been analysis and record intensity as a Bragg angle function by X–ray diffractometric system Shimadzu 6,000. The radiation source has been the Cu (Kα) with λ= 1.5405 Å wave-length, the current has been 30mA and voltage has been 40kV. The angle of the scanning 2θ has been ranged between (20⁰ and 80⁰) with speed 4 (degree per minute) with a preset time equal to 0.24sec.

FESEM is utilized for examining the morphology of two phases anatase and rutile TiO2 at different rate solutions deposited on Si substrate were carried out using (MIRA3model-TE-SCAN, (Dey Petronic Co.).

The topography surface analysis has been conducted by an atomic force microscope (AFM) (AA-3000 Scanning Probe Microscope SPM, tip NSC-35/AIBS from the Angstrom Ad- Vance Inc), the micrographs of the AFM can give information on the deposit roughness on nano- metric scales, existence of the particulates and morphology of two phases anatase and rutile TiO2

at different rate solutions deposited on glass substrate.

Hall Effect is a rich information source on conduction characteristics of the semi- conductors. The measurements of the Hall effect were performed by Van der Pauw (Ecopia HMS3000) Hall measurement systems using (0.55 Tesla) magnetic field.

3. Results and discussion

3.1. X–ray diffraction analysis

Fig. (1a) shows XRD patterns of prepared anatase Titanium dioxide phase nanostructures, which is agreement with standard peaks of anatase TiO2 in the JCPDS (standard card NO. 96-900-9087). XRD patterns of rutile TiO2 phase reveal in figure(1b), which is agreement with standard peaks of rutile TiO2 phase in the JCPDS (standard card NO. 96-900- 9084). The main peak at 25.3°, which corresponds to (101) plane according to TiO2 anatase phase and (110) plane at 27.4^o according to the TiO2 rutile phase.

(a) (b) Fig. 1. The structure of (a) Anatase, (b) Rutile TiO₂ phase.

The average crystallites size (D) of TiO2 nanostructures, with high intensity peaks for crystal planes (101) and (110) for the rutile and anatase phases respectively was estimated by Debey–Scherer’s equation [14]:

D = k λ/ β cosθ (1) λ represents the Cu-Kα radiation wave-length, θ represents the angle of Bragg’s diffraction, β represents full width at half maximum (FWHM) peak intensity, and k represents constant (approximately 0.9). Lattice constant (a, c) has been expected as well from the data of the XRD with the equation below [15]:

1 𝑑 ⁄

_ℎ𝑘𝑙²

= (ℎ

²

+ 𝑘

²

) 𝑎 ⁄

²

+ 𝑙

²

⁄ 𝑐

² (2)

where (

ℎ, 𝑘, 𝑙

) is the plane index of the crystal and

𝑑

_ℎ𝑘𝑙² is the corresponding spacing of the crystal plane. The standard and experimental interplane distances (d), FWHM, average crystallites size and Miller indices of anatase and rutile phases listed in Tables 1 and 2.

Table 1. The structural parameters for Anatase TiO₂ phase.

2θ (Deg.) FWHM (Deg.) d_hkl Exp.

(Å)

D (nm) d_hkl Std.

(Å)

hkl

25.3320 0.6264 3.5131 13.0 3.5169 (101)

37.7750 0.5900 2.3796 14.2 2.3785 (004)

47.9350 0.7729 1.8963 11.2 1.8925 (200)

53.9140 1.1938 1.6992 7.5 1.7001 (105)

54.9040 0.7838 1.6709 11.4 1.6665 (211)

62.4790 0.8746 1.4853 10.6 1.4809 (204)

68.8420 0.5925 1.3627 16.3 1.3642 (116)

70.0540 0.4585 1.3421 21.2 1.3382 (220)

75.1040 1.3307 1.2639 7.5 1.2647 (215)

56.1970 0.6064 1.6355 14.9 1.6241 (220)

63.0240 0.2517 1.4738 6.46 1.4791 (002)

69.7510 0.8414 1.3472 11.5 1.3599 (301)

3.2. FESEM

FESEM images illustrates in figure (2) for the TiO2 rutile and anatase nanoparticles at low and high magnifications. Fig. 2(a, b) reveal the rutile TiO2 nanostructure nanoflower like structure at 200nm magnifications, while for 2(c, d) the TiO2 particles, may be due to the synthesis method and its preparation conditions.

(a) (b)

(c) (d)

Fig. 2. Images of the FESEM of TiO₂ (a), (b) rutile phase and (c), (d) anatase phase.

3.3. AFM

The topography of anatase and rutile TiO2 phases were tested by AFM analysis so as to offer a large surface inspection of microstructural arrays. Figs. 3 and 4 show the images with (2D, 3D) for anatase and rutile TiO2 phase respectively.

a) b)

Fig. 3. (a) 2-D and (b) 3-D AFM images of Anatase TiO2 nanoparticles

a) b)

Fig. 4. (a) 2-D and (b) 3-D AFM images of Rutile TiO₂ nanoparticles.

Topography parameters contain average roughness, average diameter, root mean square (RMS), and roughness for samples are tabulated as can be seen in Table 3. From the results notice rutile TiO2 phase has small average diameter, high average roughness compare with anatase TiO2

phase. Roughness can be generally observed as one of the significant parameters. The surface roughness not merely describes the scattering of the light, but also provides information on the quality of the studied surface.

Table 3. Grain size, Roughness average and (RMS) Roughness (nm) of deposited Anatase and rutile TiO₂ nanoparticle.

Sample AV.Diameter

(nm)

AV. Roughness(nm) (RMS) Roughness (nm)

Anatase 76.37 4.37 5.04

rutile 57.32 12.1 14

TiO2 anatase and rutile sensors respectively, upon exposure to reducing gas H2S with a 50ppm concentration at different operating temperature values, starting from the room temperature to 473

oK with gradually increase of 323^oK.

a) b)

Fig. 5. Resistance as function of time for (a)anatase TiO₂ nanoparticle (b)rutile TiO₂ nanoparticle at RT.

The figures above illustrate the decrease of the values of the resistance when the films have been exposed to the H2S gas (Gas ON), then the value of the resistance has been increased at the gas closure (Gas OFF). The reason for such behavior may be attributed to the fact that the molecules of the oxygen on the metal oxide surface gains the electrons from metal oxides and form the ions of the oxygen. The surfaces of the metal oxide have a hole layer of accumulation for p-type and layer of the electron depletion for n-type. In the case where the reductive gases have been introduced, the resistance of n-type metal oxide gas sensor decreases because of decreasing the thickness of the electron depletion layer [16, 17].

The sensitivity can be calculated as given as [18]:

S= |(R_g- R_a) /R_a| * 100% (3)

S represents sensitivity, Ra & Rg represent the film’s electrical resistance in air and the existence of the gas, respectively.

The recovery time trec and the response time tres can be characterized as the time which is needed for the changes in the electrical resistance to take place, from base resistance which is measured in the air or the gas to 90% of the stable signal post introducing air or gas.

The response and recovery times may be calculated as follows [19]:

tres = |tgas(on) -tgas(off) |× 0.9 (4) trec =| tgas(off)- tgas(on)| × 0.9 (5) 204

204.5 205 205.5 206 206.5 207 207.5

0 30 60 90 120 150 180

Resistance(kohm)

Time (sec)

on

off

on

0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 1.04

60 90 120 150 180 210 240

Resistance(megaohm)

Time (sec)

on on

off

The response depends on the type and concentration of the gas, and the working temperature of the sensor. According to the presented results, TiO2 anatase phase sensor response to H2S at (RT, 373, and 473)^oK, while TiO2 rutile phase sensor response to H2S at (RT, 323, 373, 423, and 473)^oK as shown in figure (6), one can see high sensitivity is equal to 14.2% for TiO2

rutile while equal to 2.6% for TiO2 anatase at 373^oK.

Fig. 6. Sensitivity variation with operating temperature for anatase TiO2 and rutile sensors against H₂S gas.

TiO2 rutile has high sensitivity because of having smaller size of the grain and higher roughness surface compare with TiO2 anatase, as obtained TiO2 rutile nanoflowers promote high ratio of surface-to-volume. Since it is thin film with smaller size of the grain, which is preferred as increased ratio of the surface to volume, enhanced catalytic activity, and carrier concentration facilitate its interactions with the larger amount of the gas molecules [20].

Moreover the response time significantly longer for TiO2 rutile than TiO2 anatase based sensors as shown in table(4), may be because the flower-like sensor’s surface is of a high complexity and more advanced. The molecules of the gas require considerably more time for reaching deep in the sensor’s surface structure and be adsorbed.

From results the sensor based on a TiO2 rutile have shown the highest efficiency for reducing gas H2S detection in contrast TiO2 nano-structured sensor anatase agreement with Wojciech et al. [21].

Table 4. Response time and Recover time results of anatase TiO₂ and rutile thin films for H₂S gas at different operating temperature.

Operation Temp. RT 323°K 373°K 423°K 473°K

sample t _Res.

(s)

t _Rec.

(s)

t _Res.

(s)

t _Rec.

(s)

t _Res.

(s)

t_Rec.

(s)

t _Res.

(s)

t _Rec.

(s)

Ʈ Res.

(s)

Ʈ Rec.

(s)

TiO₂ antase phase 5.4 14.4 - - 17.1 13.5 - - 13.5 13.5 TiO₂ rutile phase 16 9 21.9 10.8 32.4 13.5 27 10 13.5 5.4

5. Conclusion

In conclusion we have demonstrated, the TiO2 nanostructures’ anatase and rutile phases were prepared by hydrothermal method successfully. We compared the sensitivity of rutile and anatase phases of TiO2. Higher sensitivity of sensor based on a TiO2 rutile than that TiO2 anatase, may be a result of the higher effective surface area. Longer TiO2 rutile delay time is possibly a result of longer required time or adsorbing more oxygen molecules for reaching initial state.

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