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

Vanadium dioxide has been of interest for thermochromic smart windows to control the near infrared radiation (NIR), but suffers limitations due to high transition temperature, low luminous transmittance and low solar modulation ability. In this paper, we report on low switching temperature and improved balance between luminous transmittance and solar modulation ability of VO₂ derived from thermal reduction of sol-gel deposited vanadium pentoxide (V₂O₅) on graphene. Thermal reduction of vanadium pentoxide (V₂O₅) was done in an innert atmosphere of flowing argon gas under normal atmospheric pressure. The optical properties of thermally reduced films were investigated using UV-VIS-NIR spectrophotometer while transition temperature was determined by measuring sheet resistance as a function of temperature. The films revealed semiconductor to metal transition after investigation. An analysis of transmittance showed an improved NIR switching efficiency of 46.86% for VO₂ on graphene compared to 41.79 for VO₂ on bare glass. Resistance drops corresponding to 2 and 3 orders of magnitude for VO₂/glass and VO₂/graphene /glass respectively were realized. VO₂ film on graphene/glass substrate had a transition temperature of 55.4 ^oC lower than 61.2^oC for VO₂ on bare glass substrate. The low Semiconductor-Metal Transition (SMT) temperature and the enhanced trade-off between luminous transmittance and solar modulation ability are noted to emanate from the increased crystallinity and nano-scale crystallites of VO₂ films enhanced by graphene matrix. The results reveal that graphene enhanced solar modulation ability and lowered transition temperature through structural ordering and nanoscale reduction of VO₂ crystallites.

Vanadium dioxide, graphene, transition temperature, smart window, near infrared radiation