Study On Synthesis And Modification Of Mixed Crystal Titanium Dioxides And Their Photocatalytic Properties For Water Spiltting To Hydrogen

As a semiconductor material with photocatalytic activity,TiO₂ can be used for photocatalysis to directly spilt water into hydrogen.However,due to the large band gap of TiO₂,it only has catalytic activity under ultraviolet light irradiation,which limits the application of TiO₂ materials.In this work,we first prepared m-TiO₂ with a mixed crystal structure,and reduced the forbidden band width by constructing heterojunctions in its structure.On this basis,the mixed crystal phase m-TiO₂ was modified by metal ions and g-C₃N₄ to reduce the recombination rate of the photoelectron-hole pairs of m-TiO₂ and enhance the photocatalytic activity of m-TiO₂ under the visible light irradiation.The results showed that:（1）Binary mixed crystal m-TiO₂ samples（A+B type）with different contents of anatase（A）and brookite（B）were synthesized by sol-gel method with tetra-n-butyl titanate as the source of titanium.At the pH value of 8.0 and the temperature of 60°C,the sample with 86%A-14%B was obtained.The photocatalytic activity of the sample reached 246 mmol/g·h under ultraviolet light irradiation.Its energy utilization efficiency was 7.5%.Binary mixed crystal m-TiO₂samples（A+R type）with different contents of anatase（A）and rutile（R）were synthesized by sol-gel method with tetra-n-butyl titanate and titanium tetrachloride as the titanium sources.At the pH value of 5.0 and the temperature of 60°C,the sample with 85%A-15%R was obtained.The photocatalytic activity of the sample reached 170 mmol/g·h under ultraviolet light irradiation.（2）The hydrothermal method was also used to synthesize binary mixed crystal m-TiO₂（A+B type）with tetra-n-butyl titanate as the titanium source.When the pH value was 7.0 and the temperature was 150°C,the sample with 85%A-15%B was obtained.The photocatalytic activity of the sample reached 223 mmol/g·h under ultraviolet light irradiation.Ternary mixed crystal m-Ti O₂（A+B+R type）was synthesized by using hydrothermal method with tetra-n-butyl titanate and titanium tetrachloride as the titanium sources.Under an optimized synthesis condition,the pH value of 1.5 and the temperature of 150°C,the sample with 56%A-40%B-4%R was obtained.The photocatalytic activity of the sample reached 250 mmol/g·h under ultraviolet light irradiation.The energy efficiency was 7.7%.（3）La-m-TiO₂ samples were prepared by introducing La³⁺into the ternary mixed crystal m-TiO₂.When the amount of La³⁺was 0.5%（molar ratio）,the photocatalytic activity of the La-m-TiO₂ sample reached 2.7 mmol/g·h under visible light irradiation,which was 50%higher than that of the parent sample.（4）Ammonium chloride modified Cl-g-C₃N₄ samples were prepared by calcination method with urea as precursor.When the mass content of ammonium chloride was 20%,the sample NCN-4 of which photocatalytic activity being 0.4 mmol/g·h,was synthesized.The Cl-C₃N₄/m-TiO₂ composite was prepared by introducing NCN-4 into the ternary mixed crystal m-TiO₂.When the amount of NCN-4 was 0.1 g,the photocatalytic activity of the Cl-C₃N₄/m-Ti O₂composite reached 4.0 mmol/g·h under visible light irradiation,which was 2.25 times that of the pre-composite sample.