Elsevier

Thin Solid Films

Volume 714, 30 November 2020, 138382
Thin Solid Films

Band gap tuning of p-type al-doped tio2 thin films for gas sensing applications

https://doi.org/10.1016/j.tsf.2020.138382Get rights and content

Highlights

  • Thermal spray pyrolysis technique is used to synthesis nano porous TiO2 films.

  • The lattice parameters of TiO2 system are found to decrease with al doping.

  • Covalent bond is formed in the synthesized sample ensured by the charge density map.

  • The electrical band energy values are matched with the optical band energy.

    Al-doped TiO2 films are found suitable for gas sensing and optoelectronic devices.

Abstract

Transparent conducting titanium dioxide (TiO2) thin films were synthesized by a spray pyrolysis technique. In this work, the effect of Al doping on the structural, morphological, topographical, optical, and electronic properties of TiO2 thin film samples was studied in detail. The deposited film shows 66 nm to 82 nm nanostructured crystallite size. The cell parameters are found in good agreement with the experimental and theoretical calculations. The pore diameters are found to be between 6 nm and 9 nm as revealed by the field emission scanning electron microscopy images. The energy dispersive X-ray analysis, spectra show that all the samples are in stoichiometric conditions. Atomic force microscope images show that the surface roughness varies from 34.72 nm to 89.83 nm. The bandgap tuning has been observed both experimentally and theoretically. The study of optical properties shows that the absorption limit of the Al-doped TiO2 sample is shifted towards the lower energy region compared with the un-doped sample. The electrical band structure energy (3.11 to 3.64 eV) values are much closer to those of the optical band structure energy (3.18 to 3.01 eV for indirect and 3.70 to 3.49 eV for direct). The charge density map ensures that the covalent bond is present in the as-deposited sample. A combined analysis of the structural, morphological, topographical, optical, and electronic properties of the compound suggests that Ti1-x AlxO2 is a potential candidate for gas sensing and photovoltaic device.

Introduction

Titanium dioxide (TiO2) is a valuable semiconductor material used widely due to potential applications in catalysis, gas sensing, photovoltaic, energy storage, medicine, to name a few [1], [2], [3], [4]. In the particular case, titanium dioxide is mainly used in the branch of photo-catalysis, because titanium dioxide has a broad bandgap (3 to 3.2 eV) and is stable at ambient temperature. The applications of titanium dioxide are mostly dependent on proper dopants such as transitions or rare-earth metals [5], [6]. Several studies have been conducted to improve the catalytic performance of titanium nanocomposites and other materials through stress, strain, and interfacial properties by doping with a transition metal, sulfide semiconductors, and carbonaceous material [7], [8], [9], [10], [11]. The characteristic of TiO2 thin film is strongly attributed to its crystal structures, morphological, and growth method [12], [13]. TiO2 thin films are used in photo-catalysis, the coating on the lenses, solar cells, and self-cleaning windows [14], [15]. In general, anatase and rutile are common phases of TiO2 but the brookite phase is rare. Brookite is the metastable phase of TiO2, made of anatase and rutile [16]. Under the laboratory condition, the brookite phase of TiO2 is more difficult to prepare. Recently, there are only a few published papers, dealing with the synthesis of the brookite phase of TiO2, a potential candidate for photovoltaic device applications [17], [18].

The n-type semiconductor TiO2 donors directly donate electrons into the conduction band (CB), leading to compensate the conduction band states and the Moss-Burstein is widening the optical band gap [19], [20], [21], [22]. The Al dopants have suppressed the position of the conduction band minimum relative to the electron affinity. A large amount of electron affinity identifies that the charge carrier is easily incorporated into the system and effectively increases carrier concentrations. However, the major drawbacks would be that TiO2 does not absorb the visible sunlight, owing to a large electrical bandgap and a high number of electron-hole recombination rates [23], [24]. Concurrently, doping with Al ion modifies the physical characteristic of TiO2 samples. The Al ions introduce new electronic energy states that cause the transport of electrons and hole combination located on the close of the Fermi-level [25]. Recently, it has been reported that doping with aluminum attenuates the band gap energy, enhances the thermal stability, modifies the conduction band to valence band, and enhances the photo-degradation efficiency [26].

TiO2 has been synthesized by various techniques, viz. sol-gel processes, the ion beam technique, reactive technique, electron beam evaporation, chemical vapor deposition, and thermal spray pyrolysis [27], [28], [29]. In our investigation, we used the thermal spray pyrolysis technique. In the present study, Al-doped TiO2 thin films were deposited onto a glass substrate. To study the theoretical calculation, we used the density functional theory applying the computational method. To see the influence of Al-doped TiO2, we adopted the plane-wave ultra-soft, a pseudo-potential method within the framework of density functional theory to simulate the electronic properties of supercell approaches, in which a Ti atom was set aside by Al atoms. In addition, the experimental results were evaluated with the theoretical results in this paper.

Section snippets

Materials

Titanium butoxide [Ti(OCH2CH2CH3)4] aluminum chloride (AlCl3•6H2O) absolute ethanol (C2H5OH) were of AR grade and purchased from Merck Germany (~99.00% purity) and deionized water as a source of additional oxygen (O) and hydrochloric acid 36% (HCl) were used to produce pure TiO2 and Al: TiO2 samples. Microscope clear glass slides of 1 mm thick (CAT No. 7101, made in China) were used in the experiment.

Preparation of pure and Al-doped TiO2 samples and studies

Microscope clear glass substrates were cleaned ultrasonically using acetone and deionized water

Surface morphological studies

The FESEM images of undoped and Al-doped TiO2 thin films are shown in Fig. 2. The Particle size is reduced with Al concentration due to the existence of dopants into the TiO2 lattice structures. Several research works have reported the reduction of particle agglomeration, with Al doping concentrations [30], [31], [32]. Porous nature is found in doped samples. The clustered collapsed particle with an average diameter of around 50 nm and the pore diameter 37.52 nm was confirmed by the scanning

Conclusions

In this paper, the synthesis of Ti1-x AlxO2 (x = 0.00, 0.02, 0.04, and 0.06) thin films via the thermal spray-spray pyrolysis method was discussed. The effect of Al-doped with TiO2 thin film samples was studied by XRD, FESEM, AFM, and UV, four-probe method, and DFT analysis. The lattice parameters were found to decrease with increasing Al dopants, which was confirmed by experimental and theoretical analysis. The crystallite size varied from 66 nm to 82 nm indicating the nanostructured

Credit Author Statement

Mohammad Nurul Islam: Investigation_Formal analysis, Jiban Podder: Supervision_Validation_Visualization, Khandker Saadat Hossain: Data curation, Suresh Sagadevan: Review & editing..

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgments

One of the authors, J. Podder is grateful to the Ministry of Science and Technology, Government of the People's Republic of Bangladesh for approval a grant. This publication is dedicated to the birth centenary of Bangabandhu Sheikh Mujibur Rahman, the ‘Father of the Nation of Bangladesh’.

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