Preparation And Electrochromic Properties Of Rare Earth Modified TiO₂ Film

Electrochromic materials can dynamically adjust the transmission and reflection properties of light,which have broad application prospects in building,automobile,military camouflage,security and other industrial fields.TiO2 is a research hotspot of electrochromic functional films because of good chemical stability,low price and convenient preparation.Using a self-made coating device,TiO2 films doped by Ce and Sm,respectively,and co-doped by Ce and Sm were prepared by sol-gel method.The effect of rare earth element on electrochromic properties of TiO2 films was studied.The electrochromic mechanism of TiO2 was discussed based on the test results.Taking butyltitanate as the main raw material and absolute ethylalcohol as solvent,uniform and transparent TiO2 films were prepared.The amorphous TiO2 film prepared with a heat treatment at 250?,3 film layers and applied voltage of ±2V with respect to the reference electrode has the most obvious color change and good electrochromic characteristics.The density of injected charge is 5.13 mC·cm-2,with a cyclic reversibility K value of 0.48.Li+injection enlarges the cell volume of anatase TiO2 and makes Fermi energy level enter the conduction band along with the valence change of Ti ions,indicating energy level transition and valence electron transfer in the LixTiO2 electrochromism process.When the amount of injected Li+ reaches 0.25,Li0.25TiO2 has the largest unit cell size and relatively good electrochromic performance.Nevertheless,as the ion storage layer for electrochromic devices,TiO2 thin film is relatively low in charge storage capacity and cyclic reversibility.The TiO2 thin film doped by Ce gradually represents the characteristics of CeO2 Cerianite with the increase of Ce doping amount.The doped Ce atoms form Ce impurity energy level at the bottom of the conduction band,which leads to the downward movement of conduction band and reduction of the forbidden bandwidth,and enhances the electrochemical reactivity of TiO2 films.In the process of electrochromic reaction,the doped Ce atoms increase the cell volume of TiO2,which is beneficial to Li+injection/extraction.The Fermi energy level of TiO2 thin film doped by Ce is also able to enter the conduction band with upward movement of the valence band,reduction of conduction band width and increase of the transition opportunity of electrons within the energy band,which largely increases the charge storage capacity and cyclic reversibility of TiO2 films.When the amount of Ce doping reaches 50%with heat treatment at 250?,the grown CeO2 crystal is relatively perfect in structure,and the charge storage capacity of film reaches its maximum value with the best cycle reversibility,with an injected charge density of 12.01 mC·cm-2 and a reversible K value of 0.9.Transmittance is 80%?85%in coloration/discoloration states,and the film is more applicable to ion storage layer for electrochromic devices.In the process of Li+injection,electrochemical reaction occurs to both Ce and Ti ions,indicating that the electrochromism reaction process contains energy level transition mechanism,electrochemical reaction mechanism and inter-valence electron transfer mechanism.The doped Sm atoms form the isolated intermediate energy level on both sides of Fermi energy level,which leads to a great drop in the forbidden bandwidth and enhances the electrochemical reactivity of the TiO2 film.In the electrochromic reaction,electrochemical reaction does not happen in Sm.The amount of Sm doping should not be too large.When Li+is injected,the overlarge Sm doping amount will lead to the reduction in Li0.25SmxTi1-xO2 cell size,increase in conduction band width,enhancement in Sm atoms electronic trap effect and reduction in electrochemical reactivity of TiO2 thin film.When Sm doping amount is 5%and heat treatment temperature is 250?,TiO2 thin film has the maximum charge storage capability with an injected charge density of 13.35mC·cm-2 and cyclic reversibility k value of 0.92.TiO2 thin film with Sm doping has low light absorption rate with a transmittance of 70%?75%in coloration state and 80%in discoloration state,which is more suitable to be used as ion storage layer without coloration effect for electrochromic devices.All results indicate that the electrochromism reaction process contains energy level transition mechanism,electrochemical reaction mechanism and inter-valence electron transfer mechanism.The joint doping of Ce and Sm leads to the existence of Ce impurity energy level and Sm impurity energy level in TiO2,in which Sm impurity energy level leads to the positive movement of valence band and Ce impurity level leads to the negative movement of valence level,and they reduce the energy required for electrons from valence band to the conduction band and enhances electrochemical reactivity of TiO2 films.When the heat treatment temperature is 250? and joint Ce and Sm doping is 2%,thin film has the maximum charge storage capacity with an injected charge density of 15.12 mC·cm-2.The TiO2 thin film with joint Ce and Sm doping has poor cyclic reversibility with an k value of 0.7.Nevertheless,the TiO2 thin film with Ce doping or Sm doping has better performance of cyclic reversibility and its k Value is able to reach 0.9.Joint doping of Ce and Sm in the TiO2 thin film results in an transmittance over 90%in coloration and discoloration states.In the Li+injection process,both Ti and Ce atoms participate electrochemical reaction in the thin film.Compared with Lio.25Ce0.05Ti0.95O2 and Li0.25Sm0.05TiO.95O2,Li0.25Ce0.05Sm0.05TiO.9O2 has the largest cell size,which increases the relocation channel of Li+ in TiO2,and the film has the best charge storage capacity.Through an accurate control of the colloid solution preparation,coating process and preparation conditions of films,rare earth modified TiO2 ion storage films with good electrochromism reversibility,strong ion storage capacity and uniform transparency were prepared,which meet the performance requirements of electrochromic devices.The doping of rare earth changes electronic stucture and electrochemical reaction process of TiO2,and then enhances its electrochemical reactivity and transmittance.The electrochromism reaction process contains energy level transition mechanism,electrochemical reaction mechanism and inter-valence electron transfer mechanism.