Title: Enhanced oxygen vacancy formation in mixed phase VOx nanorods for temperature-Independent oxygen gas sensor applications

Appu Vengattoor Raghu

PSG Institute of Advanced Studies, India


Mr. Appu Vengattoor Raghu studied Physics at the MG University, India and graduated as M.Sc. in 2012. He received his M.Phil., degree from Bharathiar University, India in 2014. He then joined the research group of Dr. P. Biji at the PSG Institute of Advanced Studies, India, for his Ph.D. Academy. He has published several international research articles in SCI(E) journals.


Herein, we report a novel and facile approach for the synthesis of oxygen deficient (intrinsic vacancies) VOx nanorods using a two-step thermal decomposition method. The role of oxygen vacancies in the performance enhancement of temperature-independent oxygen-sensing properties is demonstrated. The spectroscopic and crystallographic analysis of the material indicates the presence of mixed valent state of V2O5 and VO2 in VOx nanosystem and the metastable property of VO2 have found to play a crucial role in the temperature-independent electrical properties. A detailed study on carbon doping in VOx nanorods has been performed through XPS analysis and the formation of oxygen vacancies due to carbon dissociation was examined. The electrical property analysis of pristine VOx nanorods showed a decrease in electrical resistance by three orders of magnitude which revealed the semiconductor to metal transition of monoclinic VO2, whereas pure VOx nanorods exhibits poor sensitivity towards oxygen gas sensing.  In contrast, oxygen deficient VOx nanorods showed a very low-temperature coefficient of resistance from the temperature above 75 °C with excellent sensing performance towards oxygen gas. The oxygen deficient sites in mixed valent VOx nanorods exhibited more active sites towards adsorption of oxygen molecules (O2- and O-) on the surface. The evaluation of gas sensing properties was carried out below 500 °C which demonstrated that the sensors based on oxygen deficient VOx exhibited much higher response than that of pure VOx materials, which endows the sensor with high performances including rapid response and recovery, high sensitivity, high recycling stability and negligible temperature independent conductivity. Thus, significant vital performance is thoroughly explained in terms of adsorption-desorption mechanism and kinetics. An Elovich model was adopted for analysing the adsorption kinetics of these nano systems indicates fast adsorption mechanism within a temperature range of 100-500 °C and concentration variations of oxygen gas (100-500 ppm). These results suggest that oxygen deficient VOx nanorods can be a promising candidate for wide range temperature-independent oxygen sensor applications.

Audience take away:

• The metal oxide based chemi-resistive sensor response is strongly influenced by the working temperature causing thermal drift under temperature fluctuated conditions which adversely affect the practical applications. Our work aim to overcome these issues in gas sensors.
• This work based on oxygen vacancy rich mixed phase VOx nanorods describes overall temperature independency of oxygen sensors. The thermal induced resistance variations can be minimized by using semiconductor to metal insulator property of the VO2 nanorods. 
• The material can be considered as a highly promising candidate for temperature-independent oxygen sensor with excellent sensitivity and stability without any thermal drift under fluctuating temperature conditions