Chemiresistors Based on Li-Doped CuO–TiO2 Films

Alfio Torrisi,Jan Vaniš,Přemysl Fitl,Roman Yatskiv,Jiří Vacík,Jaroslav Otta,Antonino Cannavò,Vasily Lavrentiev,Martin Hruška,Pavel Horák,Martin Vrňata,Jan Grym,Giovanni Ceccio,Ladislav Fišer

doi:10.3390/chemosensors9090246

Abstract

Chemiresistors based on thin films of the Li-doped CuO–TiO2 heterojunctions were synthesized by a 2-step method: (i) repeated ion beam sputtering of the building elements (on the Si substrates and multisensor platforms); and (ii) thermal annealing in flowing air. The structure and composition of the films were analyzed by several methods: Rutherford Backscattering (RBS), Neutron Depth Profiling (NDP), Secondary Ion Mass Spectrometry (SIMS), and Atomic Force Microscopy (AFM), and their sensitivity to gaseous analytes was evaluated using a specific lab-made device operating in a continuous gas flow mode. The obtained results showed that the Li doping significantly increased the sensitivity of the sensors to oxidizing gases, such as NO2, O3, and Cl2, but not to reducing H2. The sensing response of the CuO–TiO2–Li chemiresistors improved with increasing Li content. For the best sensors with about 15% Li atoms, the detection limits were as follows: NO2 → 0.5 ppm, O3 → 10 ppb, and Cl2 → 0.1 ppm. The Li-doped sensors showed excellent sensing performance at a lower operating temperature (200 °C); however, even though their response time was only a few minutes, their recovery was slow (up to a few hours) and incomplete.

Highlights

The CuO–TiO2 heterogeneous films with a p–n heterojunction have attracted the attention of the scientific community as promising materials with a high application potential in gas sensing [1,2,3,4,5], and in technologies of semiconductor photocatalysis [6,7] or lithium-ion batteries [8]
The thin film samples were prepared in two steps—by ion beam sputtering of high purity Cu, Ti, and Li targets and by thermal annealing in air
The composition and elemental depth profiling were analyzed by nuclear analytical methods, namely, Neutron Depth Profiling (NDP), Rutherford Backscattering (RBS), and TOF-Secondary Ion Mass Spectrometry (SIMS), and the surface morphology by scanning probe microscopy—Atomic Force Microscopy (AFM)

Summary

Introduction

The CuO–TiO2 heterogeneous films with a p–n heterojunction have attracted the attention of the scientific community as promising materials with a high application potential in gas sensing [1,2,3,4,5], and in technologies of semiconductor photocatalysis [6,7] or lithium-ion batteries [8]. For the gas sensing, CuO–TiO2 combines the distinct advantages of the first CuO monometallic oxide, such as a low bandgap [10], with the high reactivity of the second TiO2 monometallic counterpart [11]. We have used a combined method consisting of low-energy Ar+ ion beam sputtering (IBS) and subsequent thermal annealing in air

Methods

Results

Conclusion