Gas sensors

Chapter 1: Introduction

1.6. Application of TiO 2

1.6.5. Gas sensors

The working principle of the TiO₂ gas sensors is based on the fact that the electrical conductivity changes when the TiO₂ nanomaterials are exposed to the tested gas due to the chemisorptions process of the reactive gas on the surface of the nanomaterials. The dominant electrical behavior due to the chemisorptions of hydrogen gas on the TiO2 NTs surface make a partial charge transfer to TiO2, thereby creating an electron accumulation layer that enhances the electrical conductance.¹³⁴ Fig. 1.15(a) shows a typical change in electrical resistance of TiO2 nanotubes array when exposed to different hydrogen concentration at 290

°C.¹³⁴

Ethanol gas sensing properties were studied by Wang et al.¹³⁵ The mechanisms of ethanol gas sensing on the surface of hierarchical TiO2 nanostructures were demonstrated as follows: ¹³⁶

ܱ_ଶ (gas) ↔ ܱ_ଶ (adsorption) (1.12) ܱ_ଶ (adsorption) + ݁^ି↔ ܱ_ଶ^ି (adsorption) (1.13) ܱ_ଶ^ି (adsorption) + ݁^ି↔ 2ܱ^ି (adsorption) (1.14) ܱ^ି (adsorption) + ݁^ି↔ ܱ^ଶି (adsorption) (1.15)

When TiO2 sensor is exposed to air, the oxygen species can be adsorbed on the surface of the sensor and ionized into O^- (adsorption) and O^2- (adsorption) by capturing free electrons from the conduction band of TiO2 and increase the resistance.

ܥ_ଶ ܪ_ହOH (gas) + ܱ^ଶି (adsorption) → ܥ_ଶܪ_ହܱ^ି (adsorption) + ܱܪ^ି (adsorption) (1.16) ܥ_ଶܪ_ହܱ^ି (adsorption) → (ܥ_ଶܪ_ହܱ^ି)2O (adsorption) + ܱ^ି(adsorption) + ݁^ି (1.17) When the sensor is exposed to ethanol gas, ethanol would react with O^-, O^2- and release electrons. This process lowers the resistance of the sensor [Fig. 1.15(b)].

The performance of the TiO2 sensors with respect to the sensitivity, selectivity, stability, and reproducibility is generally determined by several factors, including the nature of the reactions occurring on the oxide surface, temperature, catalytic properties of the surface, defects and electronic structure of TiO₂ nanostructures.

Fig. 1.15. Electrical response curve of (a) H2 sensor for different hydrogen concentration at operating temperature 290 °C and (b) ethanol with increasing concentrations at 350 °C. Adapted from Ref.

[134, 135].

1.7. Current challenges in TiO

2

nanostructures and focus of the present thesis

A. Understanding the formation mechanism of hydrothermally/ solvothermally synthesized different TiO₂ nanostructures:

After the first successful synthesis of titanate NTs by Kasuga et al.,¹³⁷ extensive research has been carried out on the hydrothermal growth of titanate and TiO₂ NRs, NTs and NWs by adjusting various growth parameters within the hydrothermal system to determine their effects on the nanostructure formation and resultant morphology. These parameters include reaction temperature, alkaline concentration, reaction duration, precursor phase, and crystallite size. Moreover, systematic study of solvent and temperature controlled growth of nanostructures and the exact growth mechanism of solvothermally synthesized titanate and TiO2 NTs and NWs are still topics debated extensively in the contemporary literature.

Understanding the formation mechanism and tuning the properties of TiO2 nanostructures by manipulation of intrinsic defects and doping remain a challenge for exploiting their applications in various fields. In this thesis, we have studied the formation mechanism of solvothermally synthesized 1D TiO2 NTs, NRs, NWs and nanoporous NRbs by varying the reaction temperatures, solvents and post-growth calcinations. The growth mechanism¹³⁸ of

the as-prepared nanostructures is elucidated from the systematic studies of field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) imaging.

B. Band gap tuning, visible light photocatalyst and identification of the native defects in TiO₂:

Anatase TiO₂ nanostructures having large surface area are used as a promising photocatalyst for the degradation of organic pollutants and in water splitting for hydrogen production. The band gaps of all TiO₂ polymorphs are in UV region which limits the industrial application to UV light only. Therefore, band gap engineering is essential to tune the band gap of TiO₂ into visible light so that it could be working using renewable solar light rather than using the costly, hazardous UV light. There are lots of challenges and fundamental issues related to the band gap tuning, electron-hole recombination in undoped and doped TiO2 nanostructures and the visible light photocatalytic performance, which need to be explored further. Recently, some approaches based on dopant-free, pure TiO2 phase were proposed to tune the band gap of TiO2 to visible light region which showed enhanced photocatalytic efficiency. The decisive role of surface disorder and point defects, such as oxygen vacancy (O_୴) and Ti interstitial (Ti_୧) in dictating the band gap narrowing and related application of TiO2 has been emphasized in the recent literatures, mainly through computational studies. However, experimental understanding on the actual nature of defects such as O_୴ and Ti_୧ in reduced TiO2 and its role in the visible light photocatalysis are still unclear. The concentration of the native defects typically depends on the growth conditions. However, the nature of band gap states induced by the Ti_୧ is yet to be identified experimentally. It is therefore imperative to understand the evolution of the native defects in band gap engineered TiO2 nanostructures with different growth/ processing conditions and identify the defects responsible for extended visible absorption and visible to near infrared photoluminescence in such nanostructures. In this work, through careful in-situ photoluminescence studies under controlled environment coupled with optical absorption measurement, we attempt to identify the specific defect responsible for the red shift in the absorption edge, visible and near infrared (NIR) photoluminescence (PL) emission in undoped TiO2 NRbs grown by a solvothermal technique.¹³⁹ In particular, monitoring the time evolution of the visible and NIR PL

emissions at low temperature, under high vacuum and oxygen environment, allows us to distinguish and unambiguously identify the defect states associated with O_୴, oxygen interstitial (O_୧) and Ti_୧. Our studies also reveal that these native defects are the microscopic origin of lattice expansion and contraction in undoped rutile TiO₂ nanostructures¹⁴⁰ by employing several structural and optical spectroscopic tools. The control of lattice parameters through the intrinsic defects may provide new routes to achieving novel functionalities in advanced materials that can be tailored for future technological applications.

C. Origin of ferromagnetism in TiO2-based diluted magnetic semiconductors:

In the recent times, diluted magnetic semiconductors (DMS), in particular the ferromagnetic oxides, have been at the forefront of research for spintronics and magneto-optic device applications.^{35, 36} The discovery of ferromagnetism in Co-doped TiO2 with a Curie temperature (TC) exceeding 300 K³⁶ lead the expansion of the field of DMS to oxides, leading to a rapid development of new materials and phenomena arising from a synergy of semiconductor physics and strongly correlated systems. However, in spite of several studies reported on TiO2-based DMS, there is no clear agreement about the nature and origin of the observed ferromagnetism (FM). It is being currently debated whether the observed FM in oxide DMS has anything to do with transition metal (TM) doping or might be solely related to intrinsic defects. Some reports suggested segregation and the formation of TM metal clusters as the origin of FM signal, while most recent results strongly support the intrinsic nature of FM mediated by carriers or defects. These controversial results among research groups suggest that the magnetic properties of DMS materials are critically dependent on fabrication, growth conditions, and doping agents. However, unambiguous determination of the nature of defects responsible for the observed FM remains a considerable challenge to the researchers. Till date, most of the reported FM in undoped TiO₂ was for thin films and nanoparticles (NPs). The observed FM in undoped thin films and NPs are usually weak.

Compared to thin films and nanoparticles, 1D TiO₂ nanostructures such as NWs, NRs and NRbs with high surface area can possess abundant surface defects, thus the intrinsic FM could be enhanced. Our present attempt in this work has been to enhance the magnetic moments which are solely due to intrinsic defects and explore a better understanding about the origin of observed ferromagnetism in undoped 1D TiO2 system through systematic

studies of magnetic, structural and optical properties.¹²² For further enhancement of the FM and to investigate the role of TM in the enhanced FM in TiO2 system, we studied the optical and magnetic properties of Fe doped and Cr doped TiO₂ NRbs by a solvothermal method and Co doped TiO₂ NPs by a ball milling method, and elucidate the mechanism of enhanced FM in TM (Fe, Cr and Co) doped and undoped TiO₂ systems. From the temperature dependence magnetization (M-T) measurement, we observed a high Curie temperature (T_c) of ~793 K, i.e, ferromagnetic to paramagnetic transition, for Co doped TiO₂ NPs¹⁴¹ and it is an important step for the development of practical commercial devices that can operate at and above RT.