Patrick Muvaka^1,, Waweru Mugo¹, Richard Ongeri¹, Mbiyu Ngaruiya¹

Vanadium dioxide (VO₂) thin films were prepared by thermal reduction of vanadium pentoxide (V₂O₅) films and thermochromic properties of the films were investigated. V₂O₅ films were prepared by dip-coating glass substrates at different withdrawal speeds of 5, 10, 15, and 20 mm/min. The as-deposited V₂O₅ films were annealed at different temperatures of 450, 500, 550 and 600°C in an inert environment of flowing argon gas under normal atmospheric pressure. The as-deposited V₂O₅ films were gray in color and the films changed color to golden yellow after annealing. UV-VIS-NIR spectrophotometry and sheet resistance probe were used to evaluate performance of the films. The result shows that a withdrawal speed of 5mm/min and annealing temperature of 500°C produced homogeneous VO₂ films with luminous transmittance close to the desired transmittance threshold for day-lighting. This was attributed to reduced thickness and crystallization resulting to low absorption by VO₂. Results showed that 500°C was threshold for obtaining crystalline VO₂ films. UV-VIS-NIR analysis revealed solar modulation ability of 5.2%, large transmittance change of over 35°C in the IR region and hysteresis loop of 6.0°C when going through phase transition between 25°C and 90°C. Temperature dependence of sheet resistance revealed semiconductor to metal transition behavior change of 2 orders of magnitude across transition temperature. The film showed semiconductor to metal transition temperature of 61.2°C lower than the classical value of 68°C and was attributed to crystalline structure of the films. This simple solution process followed by thermal reduction makes the VO₂ potentially useful in smart window application.

vanadium dioxide, transition temperature, near-infrared-radiation, luminous transmittance, solar modulation ability and crystallinity